--- a/testing/marionette/listener.js
+++ b/testing/marionette/listener.js
@@ -268,29 +268,33 @@ var loadListener = {
       }
     }
 
     if (loadEventExpected) {
       let startTime = new Date().getTime();
       this.start(command_id, timeout, startTime, true);
     }
 
-    try {
-      trigger();
-    } catch (e) {
+    return Task.spawn(function* () {
+      yield trigger();
+
+    }).then(val => {
+      if (!loadEventExpected) {
+        sendOk(command_id);
+      }
+
+    }).catch(err => {
       if (loadEventExpected) {
         this.stop();
       }
-      sendError(new UnknownCommandError(e.message), command_id);
+
+      // Check why we do not raise an error if err is of type Event
+      sendError(err, command_id);
       return;
-    }
-
-    if (!loadEventExpected) {
-      sendOk(command_id);
-    }
+    });
   },
 }
 
 /**
  * Called when listener is first started up.
  * The listener sends its unique window ID and its current URI to the actor.
  * If the actor returns an ID, we start the listeners. Otherwise, nothing happens.
  */