diff --git a/src/matrix/Session.js b/src/matrix/Session.js
index 3404a6aa..668677b3 100644
--- a/src/matrix/Session.js
+++ b/src/matrix/Session.js
@@ -261,7 +261,7 @@ export class Session {
                 }
             }
             this._keyBackup.get().dispose();
-            this._keyBackup.set(undefined);
+            this._keyBackup.set(null);
         }
     }