diff --git a/test/sourcemaps/samples/sourcemap-basename-without-outputname/_config.js b/test/sourcemaps/samples/sourcemap-basename-without-outputname/_config.js
index d9004f7e58..15c346bfb9 100644
--- a/test/sourcemaps/samples/sourcemap-basename-without-outputname/_config.js
+++ b/test/sourcemaps/samples/sourcemap-basename-without-outputname/_config.js
@@ -1,11 +1,26 @@
-export const component_filepath = 'src/some/deep/path/input.svelte';
 
+import {magic_string_bundle} from '../../helpers';
+
+export const component_filepath = 'src/some/deep/path/input.svelte';
 export const component_file_basename = 'input.svelte';
+export const css_file_basename = 'input.css';
 
-export default {
+const input_css = ' h1 {color: blue;}';
 
+export default {
+	preprocess: [
+		{
+			style: ({ content, filename }) => {
+				const style_to_add = `/* Filename from preprocess: ${filename} */` + input_css;
+				return  magic_string_bundle([
+					{ code: content, filename: component_file_basename },
+					{ code: style_to_add, filename: css_file_basename }
+				],component_filepath);
+			}
+		}
+	],
 	js_map_sources: [component_file_basename],
-
+	css_map_sources: [css_file_basename,component_file_basename],
 	options: {
 		filename: component_filepath
 	},
diff --git a/test/sourcemaps/samples/sourcemap-basename-without-outputname/input.svelte b/test/sourcemaps/samples/sourcemap-basename-without-outputname/input.svelte
index 4548af5548..189f28c390 100644
--- a/test/sourcemaps/samples/sourcemap-basename-without-outputname/input.svelte
+++ b/test/sourcemaps/samples/sourcemap-basename-without-outputname/input.svelte
@@ -1 +1,2 @@
 <h1>sourcemap-basename-without-outputname</h1>
+<style src="input.css">h1 {color: red;}</style>