diff --git a/src/ol/source/vectorfilesource.js b/src/ol/source/vectorfilesource.js
index d0f972800b..d003037170 100644
--- a/src/ol/source/vectorfilesource.js
+++ b/src/ol/source/vectorfilesource.js
@@ -128,11 +128,11 @@ ol.source.VectorFile.prototype.handleXhrIo_ = function(event) {
       goog.asserts.fail();
     }
     goog.dispose(xhrIo);
-    if (goog.isDef(source)) {
+    if (goog.isDefAndNotNull(source)) {
       this.readFeatures_(source);
     } else {
-      goog.asserts.fail();
       this.setState(ol.source.State.ERROR);
+      goog.asserts.fail();
     }
   } else {
     this.setState(ol.source.State.ERROR);