diff --git a/packages/automerge-repo/test/AutomergeUrl.test.ts b/packages/automerge-repo/test/AutomergeUrl.test.ts
index 17fb2f51..f2f18d7a 100644
--- a/packages/automerge-repo/test/AutomergeUrl.test.ts
+++ b/packages/automerge-repo/test/AutomergeUrl.test.ts
@@ -96,5 +96,9 @@ describe("AutomergeUrl", () => {
       const url = stringifyAutomergeUrl({ documentId: badUuidDocumentId })
       assert(isValidAutomergeUrl(url) === false)
     })
+
+    it("should return false for a documentId that is just some random type", () => {
+      assert(isValidAutomergeUrl({ foo: "bar" } as unknown) === false)
+    })
   })
 })