diff --git a/paddle/gserver/gradientmachines/GradientMachine.h b/paddle/gserver/gradientmachines/GradientMachine.h
index bc2f2f8563526aa045ea89f15152ee2d639b5774..f9c82a2bef82b4e6bcbf0c73583505d2692f3926 100644
--- a/paddle/gserver/gradientmachines/GradientMachine.h
+++ b/paddle/gserver/gradientmachines/GradientMachine.h
@@ -134,9 +134,7 @@ public:
     backward(callback);
   }
 
-  virtual Argument getLayerOutput(const std::string& layerName) {
-    return *((Argument*)nullptr);
-  }
+  virtual Argument getLayerOutput(const std::string& layerName) = 0;
 
   // see comment in Layer.h for the function with the same name
   virtual void resetState() {}