diff --git a/subprocess.hpp b/subprocess.hpp
index 0341474..41a1e3b 100755
--- a/subprocess.hpp
+++ b/subprocess.hpp
@@ -611,7 +611,7 @@ public:
 
 private:
   struct HolderBase {
-    virtual void operator()() const;
+    virtual void operator()() const = 0;
   };
   template <typename T>
   struct FuncHolder: HolderBase {