diff --git a/src/index.js b/src/index.js
index 040a5877..7c38ee1a 100644
--- a/src/index.js
+++ b/src/index.js
@@ -171,9 +171,10 @@ export default (options = {}) => {
 
         const concat = new Concat(true, fileName, '\n')
         const entries = [...extracted.values()]
+        const relativePath = path.relative(dir, file)
         const { modules, facadeModuleId } = bundle[
-          normalizePath(path.relative(dir, file))
-        ]
+          normalizePath(relativePath)
+        ] || bundle[relativePath]
 
         if (modules) {
           const moduleIds = getRecursiveImportOrder(
diff --git a/test/index.test.js b/test/index.test.js
index 0ee03c9a..05e65db5 100644
--- a/test/index.test.js
+++ b/test/index.test.js
@@ -454,3 +454,18 @@ test('augmentChunkHash', async () => {
   const barHash = barOne.fileName.split('.')[1]
   expect(barHash).not.toEqual(fooHash) // Verify that foo and bar does not hash to the same
 })
+
+test('backslashEntryFilename', async () => {
+  const outDir = fixture('dist', 'backslashEntryFilename')
+  const file = 'simple/foo.css'
+  const newBundle = await rollup({
+    input: fixture(file),
+    plugins: [postcss({ extract: true })]
+  })
+  const { output } = await newBundle.write({
+    dir: outDir,
+    entryFileNames: 'test\\foo.[hash].css'
+  })
+  const hash = output[0].fileName.split('.')[1]
+  expect(hash).toBeTruthy() // Verify that [hash] part of `foo.[hash].css` is truthy
+})