diff --git a/src/webrtc/call.ts b/src/webrtc/call.ts
index 69b72355d..1b756162f 100644
--- a/src/webrtc/call.ts
+++ b/src/webrtc/call.ts
@@ -211,8 +211,8 @@ export class CallError extends Error {
     }
 }
 
-function genCallID() {
-    return Date.now() + randomString(16);
+function genCallID(): string {
+    return Date.now().toString() + randomString(16);
 }
 
 /**