diff --git a/Logic/ElementStorage.ts b/Logic/ElementStorage.ts
index 49213b99f..8d9723f8e 100644
--- a/Logic/ElementStorage.ts
+++ b/Logic/ElementStorage.ts
@@ -38,7 +38,7 @@ export class ElementStorage {
         return es
     }
 
-    getEventSourceById(elementId): UIEventSource<any> {
+    getEventSourceById(elementId): UIEventSource<any> | undefined {
         if (elementId === undefined) {
             return undefined
         }