diff --git a/ext/bg/js/backend.js b/ext/bg/js/backend.js
index 6ce0dd603c..918f870174 100644
--- a/ext/bg/js/backend.js
+++ b/ext/bg/js/backend.js
@@ -857,7 +857,7 @@ class Backend {
                 port.postMessage({type: 'complete', data: result});
             } catch (e) {
                 if (port !== null) {
-                    port.postMessage({type: 'error', data: e});
+                    port.postMessage({type: 'error', data: errorToJson(e)});
                 }
                 cleanup();
             }