diff --git a/src/utils/test-viewmodel.ts b/src/utils/test-viewmodel.ts
index ef3d276f..990c43f8 100644
--- a/src/utils/test-viewmodel.ts
+++ b/src/utils/test-viewmodel.ts
@@ -84,6 +84,7 @@ export function getBasicRTCSession(
     getMember: (userId) => matrixRoomMembers.get(userId) ?? null,
     getMembers: () => Array.from(matrixRoomMembers.values()),
     getMembersWithMembership: () => Array.from(matrixRoomMembers.values()),
+    guessDMUserId: vitest.fn(),
     roomId: matrixRoomId,
     on: vitest
       .fn()