diff --git a/lib/core/engine.js b/lib/core/engine.js
index 561fbbbb9..af0cbd65b 100644
--- a/lib/core/engine.js
+++ b/lib/core/engine.js
@@ -168,7 +168,7 @@ class Engine {
       });
     };
     const cargo = async.cargo((tasks, callback) => {
-      const modifiedAssets = tasks.map(task => task.modifiedAsset);
+      const modifiedAssets = tasks.map(task => task.modifiedAsset).filter(asset => asset); // filter null elements
       generateCode(modifiedAssets);
       self.events.once('outputDone', callback);
     });