diff --git a/spec/unit/rust-crypto/rust-crypto.spec.ts b/spec/unit/rust-crypto/rust-crypto.spec.ts
index c21e34531..d306166de 100644
--- a/spec/unit/rust-crypto/rust-crypto.spec.ts
+++ b/spec/unit/rust-crypto/rust-crypto.spec.ts
@@ -578,9 +578,11 @@ describe("RustCrypto", () => {
     });
 
     describe("findVerificationRequestDMInProgress", () => {
-        it("returns undefined if the userId is not provided", async () => {
+        it("throws an error if the userId is not provided", async () => {
             const rustCrypto = await makeTestRustCrypto();
-            expect(rustCrypto.findVerificationRequestDMInProgress(testData.TEST_ROOM_ID)).not.toBeDefined();
+            expect(() => rustCrypto.findVerificationRequestDMInProgress(testData.TEST_ROOM_ID)).toThrow(
+                "missing userId",
+            );
         });
     });
 });
diff --git a/src/rust-crypto/rust-crypto.ts b/src/rust-crypto/rust-crypto.ts
index 8bed4c9e9..788238e6a 100644
--- a/src/rust-crypto/rust-crypto.ts
+++ b/src/rust-crypto/rust-crypto.ts
@@ -686,8 +686,7 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, RustCryptoEv
      *
      */
     public findVerificationRequestDMInProgress(roomId: string, userId?: string): VerificationRequest | undefined {
-        // TODO raise an error
-        if (!userId) return;
+        if (!userId) throw new Error("missing userId");
 
         const requests: RustSdkCryptoJs.VerificationRequest[] = this.olmMachine.getVerificationRequests(
             new RustSdkCryptoJs.UserId(userId),