diff --git a/src/matrix/calls/CallHandler.ts b/src/matrix/calls/CallHandler.ts
index 92f7314c..07ed8492 100644
--- a/src/matrix/calls/CallHandler.ts
+++ b/src/matrix/calls/CallHandler.ts
@@ -40,7 +40,7 @@ export type Options = Omit<GroupCallOptions, "emitUpdate" | "createTimeout"> & {
     clock: Clock
 };
 
-function getRoomMemberKey(roomId: string, userId: string) {
+function getRoomMemberKey(roomId: string, userId: string): string {
     return JSON.stringify(roomId)+`,`+JSON.stringify(userId);
 }