diff --git a/src/compiler/preprocess/index.ts b/src/compiler/preprocess/index.ts
index 29a5e45931..e878cdc019 100644
--- a/src/compiler/preprocess/index.ts
+++ b/src/compiler/preprocess/index.ts
@@ -191,8 +191,10 @@ function get_replacement(
 			decoded_map = decoded_sourcemap_from_generator(decoded_map);
 		}
 		// offset only segments pointing at original component source
-		const source_index = decoded_map.sources.findIndex(source => source === file_basename);
-		sourcemap_add_offset(decoded_map, get_location(offset + prefix.length), source_index);
+		const source_index = decoded_map.sources.indexOf(file_basename);
+		if (source_index !== -1) {
+			sourcemap_add_offset(decoded_map, get_location(offset + prefix.length), source_index);
+		}
 	}
 	const processed_with_map = StringWithSourcemap.from_processed(processed.code, decoded_map);
 
diff --git a/src/compiler/utils/string_with_sourcemap.ts b/src/compiler/utils/string_with_sourcemap.ts
index 3dbd0a674a..4b4be28846 100644
--- a/src/compiler/utils/string_with_sourcemap.ts
+++ b/src/compiler/utils/string_with_sourcemap.ts
@@ -15,7 +15,7 @@ function last_line_length(s: string) {
 export function sourcemap_add_offset(
 	map: DecodedSourceMap, offset: SourceLocation, source_index: number
 ) {
-	if (map.mappings.length == 0 || source_index < 0) return;
+	if (map.mappings.length == 0) return;
 	// shift lines
 	for (let line = 0; line < map.mappings.length; line++) {
 		const segment_list = map.mappings[line];
diff --git a/test/sourcemaps/samples/concat/_config.js b/test/sourcemaps/samples/concat/_config.js
new file mode 100644
index 0000000000..835b3c9e48
--- /dev/null
+++ b/test/sourcemaps/samples/concat/_config.js
@@ -0,0 +1,15 @@
+import MagicString from 'magic-string';
+import { magic_string_preprocessor_result } from '../../helpers';
+
+export default {
+	js_map_sources: ['input.svelte'],
+	preprocess: [
+		{
+			script: ({ content }) => {
+				const src = new MagicString(content);
+				src.prepend('console.log("Injected first line");\n');
+				return magic_string_preprocessor_result('input.svelte', src);
+			}
+		}
+	]
+};
diff --git a/test/sourcemaps/samples/concat/input.svelte b/test/sourcemaps/samples/concat/input.svelte
new file mode 100644
index 0000000000..18a1ac775f
--- /dev/null
+++ b/test/sourcemaps/samples/concat/input.svelte
@@ -0,0 +1,3 @@
+<script>console.log('Target')</script>
+
+<h1>Hello</h1>
\ No newline at end of file
diff --git a/test/sourcemaps/samples/concat/test.js b/test/sourcemaps/samples/concat/test.js
new file mode 100644
index 0000000000..0d7af05407
--- /dev/null
+++ b/test/sourcemaps/samples/concat/test.js
@@ -0,0 +1,9 @@
+import { assert_mapped } from '../../helpers';
+
+export function test({ input, preprocessed }) {
+	assert_mapped({
+    code: 'Target',
+		input: input.locate,
+		preprocessed
+	});
+}