diff --git a/src/mpy/sagas.test.ts b/src/mpy/sagas.test.ts
index 06260a39..78093b29 100644
--- a/src/mpy/sagas.test.ts
+++ b/src/mpy/sagas.test.ts
@@ -1,10 +1,44 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2020-2022 The Pybricks Authors
+// Copyright (c) 2020-2023 The Pybricks Authors
 
+import path from 'path';
 import { AsyncSaga } from '../../test';
 import { compile, didCompile, didFailToCompile } from './actions';
 import mpy from './sagas';
 
+const mpyCrossV5Wasm = require.resolve('@pybricks/mpy-cross-v5/build/mpy-cross.wasm');
+const mpyCrossV6Wasm = require.resolve(
+    '@pybricks/mpy-cross-v6/build/mpy-cross-v6.wasm',
+);
+
+beforeEach(() => {
+    // HACK: work around Emscripten + Webpack bugs
+    // Since we are using jsdom, emscripten thinks we are in a browser and
+    // sees that the path starts with file:// but just passes this to
+    // path.normalize() which treats file: as a windows-style drive prefix.
+    // Also, the webpack import.meta.url doesn't work correctly in the test
+    // environment either and returns a path relative to the directory where
+    // it was called rather than the node_modules/ directory. So we have to
+    // fake the normalization to get the correct path.
+    jest.spyOn(path, 'normalize').mockImplementation((p) => {
+        // NB: we can't call require.resolve() here because it would recursively
+        // call this function via path.normalize()!
+        if (p.endsWith('@pybricks/mpy-cross-v5/build/mpy-cross.wasm')) {
+            return mpyCrossV5Wasm;
+        }
+
+        if (p.endsWith('@pybricks/mpy-cross-v6/build/mpy-cross-v6.wasm')) {
+            return mpyCrossV6Wasm;
+        }
+
+        return p;
+    });
+});
+
+afterEach(() => {
+    jest.clearAllMocks();
+});
+
 test('compiler works', async () => {
     const saga = new AsyncSaga(mpy);
 
diff --git a/src/mpy/sagas.ts b/src/mpy/sagas.ts
index 7a83b932..b7b29598 100644
--- a/src/mpy/sagas.ts
+++ b/src/mpy/sagas.ts
@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2020-2022 The Pybricks Authors
+// Copyright (c) 2020-2023 The Pybricks Authors
 
 import { compile as mpyCrossCompileV5 } from '@pybricks/mpy-cross-v5';
 import { compile as mpyCrossCompileV6 } from '@pybricks/mpy-cross-v6';
@@ -54,12 +54,10 @@ function* handleCompile(action: ReturnType<typeof compile>): Generator {
                         'main.py',
                         action.script,
                         action.options,
-                        process.env.NODE_ENV === 'test'
-                            ? undefined
-                            : new URL(
-                                  '@pybricks/mpy-cross-v5/build/mpy-cross.wasm',
-                                  import.meta.url,
-                              ).toString(),
+                        new URL(
+                            '@pybricks/mpy-cross-v5/build/mpy-cross.wasm',
+                            import.meta.url,
+                        ).toString(),
                     ),
                 );
                 if (result.status === 0 && result.mpy) {
@@ -77,12 +75,10 @@ function* handleCompile(action: ReturnType<typeof compile>): Generator {
                         'main.py',
                         action.script,
                         action.options,
-                        process.env.NODE_ENV === 'test'
-                            ? undefined
-                            : new URL(
-                                  '@pybricks/mpy-cross-v6/build/mpy-cross-v6.wasm',
-                                  import.meta.url,
-                              ).toString(),
+                        new URL(
+                            '@pybricks/mpy-cross-v6/build/mpy-cross-v6.wasm',
+                            import.meta.url,
+                        ).toString(),
                     ),
                 );
                 if (result.status === 0 && result.mpy) {
@@ -188,12 +184,10 @@ function* handleCompileMulti6(): Generator {
                 py.path,
                 py.contents,
                 undefined,
-                process.env.NODE_ENV === 'test'
-                    ? undefined
-                    : new URL(
-                          '@pybricks/mpy-cross-v6/build/mpy-cross-v6.wasm',
-                          import.meta.url,
-                      ).toString(),
+                new URL(
+                    '@pybricks/mpy-cross-v6/build/mpy-cross-v6.wasm',
+                    import.meta.url,
+                ).toString(),
             ),
         );