diff --git a/src/compile/Component.ts b/src/compile/Component.ts
index 37abcb591a..0afb903b29 100644
--- a/src/compile/Component.ts
+++ b/src/compile/Component.ts
@@ -22,6 +22,7 @@ import fuzzymatch from '../utils/fuzzymatch';
 import { remove_indentation, add_indentation } from '../utils/indentation';
 import getObject from '../utils/getObject';
 import deindent from '../utils/deindent';
+import globalWhitelist from '../utils/globalWhitelist';
 
 type Meta = {
 	namespace?: string;
@@ -879,11 +880,14 @@ export default class Component {
 		return `ctx.${name}`;
 	}
 
-	warn_if_undefined(node, template_scope: TemplateScope) {
+	warn_if_undefined(node, template_scope: TemplateScope, allow_implicit?: boolean) {
 		const { name } = node;
-		if (this.module_scope && this.module_scope.declarations.has(name)) return;
+
+		if (allow_implicit && !this.instance_script) return;
 		if (this.instance_scope && this.instance_scope.declarations.has(name)) return;
+		if (this.module_scope && this.module_scope.declarations.has(name)) return;
 		if (template_scope.names.has(name)) return;
+		if (globalWhitelist.has(name)) return;
 
 		this.warn(node, {
 			code: 'missing-declaration',
diff --git a/src/compile/nodes/shared/Expression.ts b/src/compile/nodes/shared/Expression.ts
index 6a6598224e..0a0c4c6cb6 100644
--- a/src/compile/nodes/shared/Expression.ts
+++ b/src/compile/nodes/shared/Expression.ts
@@ -141,7 +141,7 @@ export default class Expression {
 				}
 
 				if (isReference(node, parent)) {
-					const { name } = flattenReference(node);
+					const { name, nodes } = flattenReference(node);
 
 					if (scope.has(name)) return;
 					if (globalWhitelist.has(name) && component.declarations.indexOf(name) === -1) return;
@@ -155,6 +155,8 @@ export default class Expression {
 					} else {
 						add_dependency(name);
 						component.template_references.add(name);
+
+						component.warn_if_undefined(nodes[0], template_scope, true);
 					}
 
 					this.skip();
diff --git a/src/parse/read/script.ts b/src/parse/read/script.ts
index 0518bc51a6..b6d6cd53a8 100644
--- a/src/parse/read/script.ts
+++ b/src/parse/read/script.ts
@@ -35,8 +35,6 @@ export default function readScript(parser: Parser, start: number, attributes: No
 		parser.acornError(err);
 	}
 
-	if (!ast.body.length) return null;
-
 	ast.start = scriptStart;
 	return {
 		start,
diff --git a/test/parser/samples/script-comment-only/output.json b/test/parser/samples/script-comment-only/output.json
index 5e18e97e74..b04d38633d 100644
--- a/test/parser/samples/script-comment-only/output.json
+++ b/test/parser/samples/script-comment-only/output.json
@@ -21,5 +21,18 @@
 		]
 	},
 	"css": [],
-	"js": []
+	"js": [
+		{
+			"start": 0,
+			"end": 43,
+			"attributes": [],
+			"content": {
+				"type": "Program",
+				"start": 8,
+				"end": 34,
+				"body": [],
+				"sourceType": "module"
+			}
+		}
+	]
 }
\ No newline at end of file
diff --git a/test/validator/samples/undefined-value/input.html b/test/validator/samples/undefined-value/input.html
new file mode 100644
index 0000000000..03dda2c44f
--- /dev/null
+++ b/test/validator/samples/undefined-value/input.html
@@ -0,0 +1,6 @@
+<script>
+	// script block prevents auto-declaration
+</script>
+
+<p>{potato}</p>
+<p>{Math.max(1, 2)}</p>
\ No newline at end of file
diff --git a/test/validator/samples/undefined-value/warnings.json b/test/validator/samples/undefined-value/warnings.json
new file mode 100644
index 0000000000..c1813e6ab0
--- /dev/null
+++ b/test/validator/samples/undefined-value/warnings.json
@@ -0,0 +1,15 @@
+[{
+	"code": "missing-declaration",
+	"message": "'potato' is not defined",
+	"pos": 67,
+	"start": {
+		"line": 5,
+		"column": 4,
+		"character": 67
+	},
+	"end": {
+		"line": 5,
+		"column": 10,
+		"character": 73
+	}
+}]
\ No newline at end of file