diff --git a/package.json b/package.json
index 27e5103..a57c3cc 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@profullstack/player",
-  "version": "0.3.0",
+  "version": "0.3.1",
   "description": "One web player for every source a Profullstack site serves: MP4, HLS, MPEG-2 transport streams and audio, with one control bar, on desktop, mobile, PWA and television.",
   "keywords": [
     "video",
diff --git a/src/engines/codecs.ts b/src/engines/codecs.ts
index e43746d..3f9e6f7 100644
--- a/src/engines/codecs.ts
+++ b/src/engines/codecs.ts
@@ -85,7 +85,12 @@ export function codecName(codec: string | null | undefined): string | null {
  * handed is the one that has to pass, and only that one is ever true here anyway.
  */
 export function mseCandidates(kind: 'video' | 'audio', codec: string | undefined): string[] {
-  if (!codec) return [];
+  // `kind` is typed, and that protects TypeScript callers only. This module is
+  // imported by two plain-JavaScript client bundles where a missing kind is an
+  // ordinary runtime value, and the original guarded both -- dropping it in the
+  // port turned `mseCandidates(null, 'mp3')` from "nothing to ask" into a
+  // question about `null/mp4`. Caught by genrewatch's own suite on adoption.
+  if (!codec || !kind) return [];
 
   if (kind === 'audio') {
     /*
diff --git a/test/codecs.test.ts b/test/codecs.test.ts
index 5715db2..432ce38 100644
--- a/test/codecs.test.ts
+++ b/test/codecs.test.ts
@@ -35,8 +35,12 @@ describe('mseCandidates', () => {
     expect(mseCandidates('audio', 'ac-3')).toEqual(['audio/mp4; codecs="ac-3"']);
   });
 
-  it('has nothing to ask when there is no codec', () => {
+  it('has nothing to ask when there is no codec, or no kind', () => {
     expect(mseCandidates('audio', undefined)).toEqual([]);
+    // Regression: `kind` is typed, which protects TypeScript callers and not
+    // the two JavaScript bundles that import this. Without the runtime guard,
+    // a missing kind asked MSE about `null/mp4`.
+    expect(mseCandidates(null as unknown as 'audio', 'mp3')).toEqual([]);
   });
 });