diff --git a/frontend/src/stores.js b/frontend/src/stores.js
index d47f378..826e779 100644
--- a/frontend/src/stores.js
+++ b/frontend/src/stores.js
@@ -301,11 +301,14 @@ class OverlayStore extends Store {
             toast: null,
             login: null,
             createAccount: null,
-            settings: null
+            editMessage: null,
+            settings: null,
         }, "OverlayStore");
     }
 
     open(name, props={}) {
+        if (this.value[name] === undefined)
+            throw new Error(`OverlayStore.open: tried to open unknown overlay with name '${name}' (undefined in overlay map)`);
         this.value[name] = props;
         this.updated();
     }