--- a/devtools/shared/deprecated-sync-thenables.js
+++ b/devtools/shared/deprecated-sync-thenables.js
@@ -56,16 +56,19 @@ function defer() {
 
       if (observers) {
         observers.push({ resolve: resolve, reject: reject });
       } else {
         result.then(resolve, reject);
       }
 
       return deferred.promise;
+    },
+    catch: function (callback) {
+      return this.then(null, callback);
     }
   };
 
   var deferred = {
     promise: promise,
     resolve: function resolve(value) {
       if (!result) {
         result = isPromise(value) ? value : fulfilled(value);