diff --git a/workers/skill-harness/src/skillgen-opt-ins.ts b/workers/skill-harness/src/skillgen-opt-ins.ts
index a0f15677..925c30f6 100644
--- a/workers/skill-harness/src/skillgen-opt-ins.ts
+++ b/workers/skill-harness/src/skillgen-opt-ins.ts
@@ -17,8 +17,9 @@ export async function readSkillgenOptIns(input: {
   fetch: typeof fetch
 }): Promise<{ _tag: 'Ok', repositories: Set<string> } | { _tag: 'Unavailable', status: number }> {
   const optedIn = new Set<string>()
+  const fetcher = input.fetch
   for (let start = 0; start < input.repositories.length; start += BATCH) {
-    const response = await input.fetch(new URL('/api/internal/skillgen/opt-ins', input.siteUrl), {
+    const response = await fetcher(new URL('/api/internal/skillgen/opt-ins', input.siteUrl), {
       method: 'POST',
       redirect: 'manual',
       signal: AbortSignal.timeout(10_000),
diff --git a/workers/skill-harness/test/skillgen-opt-ins.test.ts b/workers/skill-harness/test/skillgen-opt-ins.test.ts
new file mode 100644
index 00000000..62930125
--- /dev/null
+++ b/workers/skill-harness/test/skillgen-opt-ins.test.ts
@@ -0,0 +1,19 @@
+import { expect, it } from 'vitest'
+import { readSkillgenOptIns } from '../src/skillgen-opt-ins'
+
+it('reads opt-ins with a fetch function that rejects an object receiver', async () => {
+  const fetcher: typeof fetch = async function (this: unknown) {
+    if (this !== undefined && this !== globalThis)
+      throw new TypeError('Illegal invocation')
+    return Response.json({ repositories: ['Maintainer/opted'] })
+  }
+
+  const result = await readSkillgenOptIns({
+    siteUrl: 'https://skilld.dev',
+    token: 'site-token',
+    repositories: ['Maintainer/opted'],
+    fetch: fetcher,
+  })
+
+  expect(result).toEqual({ _tag: 'Ok', repositories: new Set(['maintainer/opted']) })
+})