diff --git a/src/compiler/compile/render_dom/wrappers/Element/Attribute.ts b/src/compiler/compile/render_dom/wrappers/Element/Attribute.ts
index aff06e4150..7c174acb46 100644
--- a/src/compiler/compile/render_dom/wrappers/Element/Attribute.ts
+++ b/src/compiler/compile/render_dom/wrappers/Element/Attribute.ts
@@ -5,6 +5,7 @@ import ElementWrapper from './index';
 import { stringify } from '../../../utils/stringify';
 import deindent from '../../../utils/deindent';
 import Expression from '../../../nodes/shared/Expression';
+import Text from '../../../nodes/Text';
 
 export default class AttributeWrapper {
 	node: Attribute;
@@ -84,26 +85,13 @@ export default class AttributeWrapper {
 				value = (this.node.chunks[0] as Expression).render(block);
 			} else {
 				// '{foo} {bar}' — treat as string concatenation
-				value =
-					(this.node.chunks[0].type === 'Text' ? '' : `"" + `) +
-					this.node.chunks
-						.map((chunk) => {
-							if (chunk.type === 'Text') {
-								return stringify(chunk.data);
-							} else {
-								const renderedChunk = chunk.render();
-								if (this.node.name === 'class') {
-									return chunk.get_precedence() <= 13
-										? `(${renderedChunk})`
-									  : `(${renderedChunk} || '')`;
-								} else {
-									return chunk.get_precedence() <= 13
-									  ? `(${renderedChunk})`
-									  : renderedChunk;
-								}
-							}
-						})
-						.join(' + ');
+				const attrPrefix = this.node.chunks[0].type === 'Text' ? '' : `"" + `;
+
+				const attrText = this.node.name === 'class'
+					? this.get_class_name_text()
+					: this.get_attr_text();
+
+				value = `${attrPrefix}${attrText}`;
 			}
 
 			const is_select_value_attribute =
@@ -217,6 +205,41 @@ export default class AttributeWrapper {
 		}
 	}
 
+	get_class_name_text() {
+		const isStyled = this.node.chunks
+			.filter((chunk) => chunk.type === 'Text')
+			.some((chunk: Text) => !chunk.start && !chunk.end);
+
+		const classNameStringArray = this.render_attr();
+
+
+		if (!isStyled || classNameStringArray.length !== 2) {
+			return classNameStringArray.join(' + ');
+		}
+
+		const targetToken = 0;
+		return classNameStringArray
+			.map((token, index) => index === targetToken ? `@class_name_resolver(${token})` : token)
+			.join(' + ');
+	}
+
+	get_attr_text() {
+		return this.render_attr().join(' + ');
+	}
+
+	render_attr() {
+		return this.node.chunks.map((chunk) => {
+			if (chunk.type === 'Text') {
+				return stringify(chunk.data);
+			}
+
+			const renderedChunk = chunk.render();
+			return chunk.get_precedence() <= 13
+				? `(${renderedChunk})`
+				: renderedChunk;
+		});
+	}
+
 	stringify() {
 		if (this.node.is_true) return '';
 
diff --git a/src/runtime/internal/utils.ts b/src/runtime/internal/utils.ts
index 08410ec33a..1aec30ef2f 100644
--- a/src/runtime/internal/utils.ts
+++ b/src/runtime/internal/utils.ts
@@ -89,3 +89,7 @@ export function once(fn) {
 		fn.call(this, ...args);
 	};
 }
+
+export function class_name_resolver(nextClassName) {
+	return nextClassName == undefined ? '' : nextClassName;
+}
\ No newline at end of file
diff --git a/test/runtime/samples/attribute-false/_config.js b/test/runtime/samples/attribute-false/_config.js
new file mode 100644
index 0000000000..0f37392806
--- /dev/null
+++ b/test/runtime/samples/attribute-false/_config.js
@@ -0,0 +1,5 @@
+export default {
+	skip_if_ssr: true,
+
+	html: `<div class="false"></div>`,
+};
diff --git a/test/runtime/samples/attribute-false/main.svelte b/test/runtime/samples/attribute-false/main.svelte
new file mode 100644
index 0000000000..ca78020a39
--- /dev/null
+++ b/test/runtime/samples/attribute-false/main.svelte
@@ -0,0 +1 @@
+<div class={false}></div>
diff --git a/test/runtime/samples/attribute-null-classname-no-style/_config.js b/test/runtime/samples/attribute-null-classname-no-style/_config.js
new file mode 100644
index 0000000000..e2098430e4
--- /dev/null
+++ b/test/runtime/samples/attribute-null-classname-no-style/_config.js
@@ -0,0 +1,44 @@
+export default {
+	skip_if_ssr: true,
+
+	props: {
+		testName: "testClassName"
+	},
+
+	html: `<div class="testClassName"></div>`,
+
+	test({ assert, component, target }) {
+		const div = target.querySelector('div');
+		assert.equal(div.className, 'testClassName');
+
+		component.testName = null;
+		assert.equal(div.className, '');
+
+		component.testName = undefined;
+		assert.equal(div.className, '');
+
+		component.testName = undefined + '';
+		assert.equal(div.className, 'undefined');
+
+		component.testName = null + '';
+		assert.equal(div.className, 'null');
+
+		component.testName = 1;
+		assert.equal(div.className, '1');
+
+		component.testName = 0;
+		assert.equal(div.className, '0');
+
+		component.testName = false;
+		assert.equal(div.className, 'false');
+
+		component.testName = true;
+		assert.equal(div.className, 'true');
+
+		component.testName = {};
+		assert.equal(div.className, '[object Object]');
+
+		component.testName = '';
+		assert.equal(div.className, '');
+	}
+};
diff --git a/test/runtime/samples/attribute-null-classname-no-style/main.svelte b/test/runtime/samples/attribute-null-classname-no-style/main.svelte
new file mode 100644
index 0000000000..8dfcd7af54
--- /dev/null
+++ b/test/runtime/samples/attribute-null-classname-no-style/main.svelte
@@ -0,0 +1,5 @@
+<script>
+	export let testName;
+</script>
+
+<div class={testName}></div>
diff --git a/test/runtime/samples/attribute-null-classname/_config.js b/test/runtime/samples/attribute-null-classname-with-style/_config.js
similarity index 70%
rename from test/runtime/samples/attribute-null-classname/_config.js
rename to test/runtime/samples/attribute-null-classname-with-style/_config.js
index 856bd8bcd0..635dd058ca 100644
--- a/test/runtime/samples/attribute-null-classname/_config.js
+++ b/test/runtime/samples/attribute-null-classname-with-style/_config.js
@@ -26,6 +26,18 @@ export default {
 		component.testName = 1;
 		assert.equal(div.className, '1 svelte-x1o6ra');
 
+		component.testName = 0;
+		assert.equal(div.className, '0 svelte-x1o6ra');
+
+		component.testName = false;
+		assert.equal(div.className, 'false svelte-x1o6ra');
+
+		component.testName = true;
+		assert.equal(div.className, 'true svelte-x1o6ra');
+
+		component.testName = {};
+		assert.equal(div.className, '[object Object] svelte-x1o6ra');
+
 		component.testName = '';
 		assert.equal(div.className, ' svelte-x1o6ra');
 	}
diff --git a/test/runtime/samples/attribute-null-classname/main.svelte b/test/runtime/samples/attribute-null-classname-with-style/main.svelte
similarity index 100%
rename from test/runtime/samples/attribute-null-classname/main.svelte
rename to test/runtime/samples/attribute-null-classname-with-style/main.svelte
diff --git a/test/runtime/samples/attribute-null-classnames-no-style/_config.js b/test/runtime/samples/attribute-null-classnames-no-style/_config.js
new file mode 100644
index 0000000000..6a734efff5
--- /dev/null
+++ b/test/runtime/samples/attribute-null-classnames-no-style/_config.js
@@ -0,0 +1,47 @@
+export default {
+	skip_if_ssr: true,
+
+	props: {
+		testName1: "test1",
+		testName2: "test2",
+	},
+
+	html: `<div class="test1test2"></div>`,
+
+	test({ assert, component, target }) {
+		const div = target.querySelector('div');
+		assert.equal(div.className, 'test1test2');
+
+		component.testName1 = null;
+		component.testName2 = null;
+		assert.equal(div.className, '0');
+
+		component.testName1 = null;
+		component.testName2 = "test";
+		assert.equal(div.className, 'nulltest');
+
+		component.testName1 = undefined;
+		component.testName2 = "test";
+		assert.equal(div.className, 'undefinedtest');
+
+		component.testName1 = undefined;
+		component.testName2 = undefined;
+		assert.equal(div.className, 'NaN');
+
+		component.testName1 = null;
+		component.testName2 = 1;
+		assert.equal(div.className, '1');
+
+		component.testName1 = undefined;
+		component.testName2 = 1;
+		assert.equal(div.className, 'NaN');
+
+		component.testName1 = null;
+		component.testName2 = 0;
+		assert.equal(div.className, '0');
+
+		component.testName1 = undefined;
+		component.testName2 = 0;
+		assert.equal(div.className, 'NaN');
+	}
+};
diff --git a/test/runtime/samples/attribute-null-classnames-no-style/main.svelte b/test/runtime/samples/attribute-null-classnames-no-style/main.svelte
new file mode 100644
index 0000000000..f8cf7d803e
--- /dev/null
+++ b/test/runtime/samples/attribute-null-classnames-no-style/main.svelte
@@ -0,0 +1,6 @@
+<script>
+	export let testName1;
+	export let testName2;
+</script>
+
+<div class={testName1 + testName2}></div>
diff --git a/test/runtime/samples/attribute-null-classnames/_config.js b/test/runtime/samples/attribute-null-classnames-with-style/_config.js
similarity index 81%
rename from test/runtime/samples/attribute-null-classnames/_config.js
rename to test/runtime/samples/attribute-null-classnames-with-style/_config.js
index 6eca569250..22f7a4d7e2 100644
--- a/test/runtime/samples/attribute-null-classnames/_config.js
+++ b/test/runtime/samples/attribute-null-classnames-with-style/_config.js
@@ -35,5 +35,13 @@ export default {
 		component.testName1 = undefined;
 		component.testName2 = 1;
 		assert.equal(div.className, 'NaN svelte-x1o6ra');
+
+		component.testName1 = null;
+		component.testName2 = 0;
+		assert.equal(div.className, '0 svelte-x1o6ra');
+
+		component.testName1 = undefined;
+		component.testName2 = 0;
+		assert.equal(div.className, 'NaN svelte-x1o6ra');
 	}
 };
diff --git a/test/runtime/samples/attribute-null-classnames/main.svelte b/test/runtime/samples/attribute-null-classnames-with-style/main.svelte
similarity index 100%
rename from test/runtime/samples/attribute-null-classnames/main.svelte
rename to test/runtime/samples/attribute-null-classnames-with-style/main.svelte
diff --git a/test/runtime/samples/attribute-null-func-classname-no-style/_config.js b/test/runtime/samples/attribute-null-func-classname-no-style/_config.js
new file mode 100644
index 0000000000..e2098430e4
--- /dev/null
+++ b/test/runtime/samples/attribute-null-func-classname-no-style/_config.js
@@ -0,0 +1,44 @@
+export default {
+	skip_if_ssr: true,
+
+	props: {
+		testName: "testClassName"
+	},
+
+	html: `<div class="testClassName"></div>`,
+
+	test({ assert, component, target }) {
+		const div = target.querySelector('div');
+		assert.equal(div.className, 'testClassName');
+
+		component.testName = null;
+		assert.equal(div.className, '');
+
+		component.testName = undefined;
+		assert.equal(div.className, '');
+
+		component.testName = undefined + '';
+		assert.equal(div.className, 'undefined');
+
+		component.testName = null + '';
+		assert.equal(div.className, 'null');
+
+		component.testName = 1;
+		assert.equal(div.className, '1');
+
+		component.testName = 0;
+		assert.equal(div.className, '0');
+
+		component.testName = false;
+		assert.equal(div.className, 'false');
+
+		component.testName = true;
+		assert.equal(div.className, 'true');
+
+		component.testName = {};
+		assert.equal(div.className, '[object Object]');
+
+		component.testName = '';
+		assert.equal(div.className, '');
+	}
+};
diff --git a/test/runtime/samples/attribute-null-func-classname-no-style/main.svelte b/test/runtime/samples/attribute-null-func-classname-no-style/main.svelte
new file mode 100644
index 0000000000..e2754cd421
--- /dev/null
+++ b/test/runtime/samples/attribute-null-func-classname-no-style/main.svelte
@@ -0,0 +1,9 @@
+<script>
+	export let testName;
+
+	function myHelper(testName) {
+		return testName;
+	}
+</script>
+
+<div class={myHelper(testName)}></div>
diff --git a/test/runtime/samples/attribute-null-func-classname-with-style/_config.js b/test/runtime/samples/attribute-null-func-classname-with-style/_config.js
new file mode 100644
index 0000000000..635dd058ca
--- /dev/null
+++ b/test/runtime/samples/attribute-null-func-classname-with-style/_config.js
@@ -0,0 +1,44 @@
+export default {
+	skip_if_ssr: true,
+
+	props: {
+		testName: "testClassName"
+	},
+
+	html: `<div class="testClassName svelte-x1o6ra"></div>`,
+
+	test({ assert, component, target }) {
+		const div = target.querySelector('div');
+		assert.equal(div.className, 'testClassName svelte-x1o6ra');
+
+		component.testName = null;
+		assert.equal(div.className, ' svelte-x1o6ra');
+
+		component.testName = undefined;
+		assert.equal(div.className, ' svelte-x1o6ra');
+
+		component.testName = undefined + '';
+		assert.equal(div.className, 'undefined svelte-x1o6ra');
+
+		component.testName = null + '';
+		assert.equal(div.className, 'null svelte-x1o6ra');
+
+		component.testName = 1;
+		assert.equal(div.className, '1 svelte-x1o6ra');
+
+		component.testName = 0;
+		assert.equal(div.className, '0 svelte-x1o6ra');
+
+		component.testName = false;
+		assert.equal(div.className, 'false svelte-x1o6ra');
+
+		component.testName = true;
+		assert.equal(div.className, 'true svelte-x1o6ra');
+
+		component.testName = {};
+		assert.equal(div.className, '[object Object] svelte-x1o6ra');
+
+		component.testName = '';
+		assert.equal(div.className, ' svelte-x1o6ra');
+	}
+};
diff --git a/test/runtime/samples/attribute-null-func-classname-with-style/main.svelte b/test/runtime/samples/attribute-null-func-classname-with-style/main.svelte
new file mode 100644
index 0000000000..bcb858ab8d
--- /dev/null
+++ b/test/runtime/samples/attribute-null-func-classname-with-style/main.svelte
@@ -0,0 +1,15 @@
+<script>
+	export let testName;
+
+	function myHelper(testName) {
+		return testName;
+	}
+</script>
+
+<style>
+  div {
+    color: purple;
+  }
+</style>
+
+<div class={myHelper(testName)}></div>
diff --git a/test/runtime/samples/attribute-null-func-classnames-no-style/_config.js b/test/runtime/samples/attribute-null-func-classnames-no-style/_config.js
new file mode 100644
index 0000000000..6a734efff5
--- /dev/null
+++ b/test/runtime/samples/attribute-null-func-classnames-no-style/_config.js
@@ -0,0 +1,47 @@
+export default {
+	skip_if_ssr: true,
+
+	props: {
+		testName1: "test1",
+		testName2: "test2",
+	},
+
+	html: `<div class="test1test2"></div>`,
+
+	test({ assert, component, target }) {
+		const div = target.querySelector('div');
+		assert.equal(div.className, 'test1test2');
+
+		component.testName1 = null;
+		component.testName2 = null;
+		assert.equal(div.className, '0');
+
+		component.testName1 = null;
+		component.testName2 = "test";
+		assert.equal(div.className, 'nulltest');
+
+		component.testName1 = undefined;
+		component.testName2 = "test";
+		assert.equal(div.className, 'undefinedtest');
+
+		component.testName1 = undefined;
+		component.testName2 = undefined;
+		assert.equal(div.className, 'NaN');
+
+		component.testName1 = null;
+		component.testName2 = 1;
+		assert.equal(div.className, '1');
+
+		component.testName1 = undefined;
+		component.testName2 = 1;
+		assert.equal(div.className, 'NaN');
+
+		component.testName1 = null;
+		component.testName2 = 0;
+		assert.equal(div.className, '0');
+
+		component.testName1 = undefined;
+		component.testName2 = 0;
+		assert.equal(div.className, 'NaN');
+	}
+};
diff --git a/test/runtime/samples/attribute-null-func-classnames-no-style/main.svelte b/test/runtime/samples/attribute-null-func-classnames-no-style/main.svelte
new file mode 100644
index 0000000000..e1cdf51412
--- /dev/null
+++ b/test/runtime/samples/attribute-null-func-classnames-no-style/main.svelte
@@ -0,0 +1,11 @@
+<script>
+	export let testName1;
+  export let testName2;
+  
+
+	function myHelper(testName) {
+		return testName;
+	}
+</script>
+
+<div class={myHelper(testName1) + myHelper(testName2)}></div>
diff --git a/test/runtime/samples/attribute-null-func-classnames-with-style/_config.js b/test/runtime/samples/attribute-null-func-classnames-with-style/_config.js
new file mode 100644
index 0000000000..22f7a4d7e2
--- /dev/null
+++ b/test/runtime/samples/attribute-null-func-classnames-with-style/_config.js
@@ -0,0 +1,47 @@
+export default {
+	skip_if_ssr: true,
+
+	props: {
+		testName1: "test1",
+		testName2: "test2",
+	},
+
+	html: `<div class="test1test2 svelte-x1o6ra"></div>`,
+
+	test({ assert, component, target }) {
+		const div = target.querySelector('div');
+		assert.equal(div.className, 'test1test2 svelte-x1o6ra');
+
+		component.testName1 = null;
+		component.testName2 = null;
+		assert.equal(div.className, '0 svelte-x1o6ra');
+
+		component.testName1 = null;
+		component.testName2 = "test";
+		assert.equal(div.className, 'nulltest svelte-x1o6ra');
+
+		component.testName1 = undefined;
+		component.testName2 = "test";
+		assert.equal(div.className, 'undefinedtest svelte-x1o6ra');
+
+		component.testName1 = undefined;
+		component.testName2 = undefined;
+		assert.equal(div.className, 'NaN svelte-x1o6ra');
+
+		component.testName1 = null;
+		component.testName2 = 1;
+		assert.equal(div.className, '1 svelte-x1o6ra');
+
+		component.testName1 = undefined;
+		component.testName2 = 1;
+		assert.equal(div.className, 'NaN svelte-x1o6ra');
+
+		component.testName1 = null;
+		component.testName2 = 0;
+		assert.equal(div.className, '0 svelte-x1o6ra');
+
+		component.testName1 = undefined;
+		component.testName2 = 0;
+		assert.equal(div.className, 'NaN svelte-x1o6ra');
+	}
+};
diff --git a/test/runtime/samples/attribute-null-func-classnames-with-style/main.svelte b/test/runtime/samples/attribute-null-func-classnames-with-style/main.svelte
new file mode 100644
index 0000000000..af43778365
--- /dev/null
+++ b/test/runtime/samples/attribute-null-func-classnames-with-style/main.svelte
@@ -0,0 +1,17 @@
+<script>
+	export let testName1;
+  export let testName2;
+  
+
+	function myHelper(testName) {
+		return testName;
+	}
+</script>
+
+<style>
+  div {
+    color: purple;
+  }
+</style>
+
+<div class={myHelper(testName1) + myHelper(testName2)}></div>
diff --git a/test/runtime/samples/attribute-null/_config.js b/test/runtime/samples/attribute-null/_config.js
new file mode 100644
index 0000000000..938d525cf6
--- /dev/null
+++ b/test/runtime/samples/attribute-null/_config.js
@@ -0,0 +1,5 @@
+export default {
+	skip_if_ssr: true,
+
+	html: `<div></div>`,
+};
diff --git a/test/runtime/samples/attribute-null/main.svelte b/test/runtime/samples/attribute-null/main.svelte
new file mode 100644
index 0000000000..9b7f48eda3
--- /dev/null
+++ b/test/runtime/samples/attribute-null/main.svelte
@@ -0,0 +1 @@
+<div class={null}></div>
diff --git a/test/runtime/samples/attribute-undefined/_config.js b/test/runtime/samples/attribute-undefined/_config.js
new file mode 100644
index 0000000000..938d525cf6
--- /dev/null
+++ b/test/runtime/samples/attribute-undefined/_config.js
@@ -0,0 +1,5 @@
+export default {
+	skip_if_ssr: true,
+
+	html: `<div></div>`,
+};
diff --git a/test/runtime/samples/attribute-undefined/main.svelte b/test/runtime/samples/attribute-undefined/main.svelte
new file mode 100644
index 0000000000..5108aa873f
--- /dev/null
+++ b/test/runtime/samples/attribute-undefined/main.svelte
@@ -0,0 +1 @@
+<div class={undefined}></div>