diff --git a/src/interpreter.hpp b/src/interpreter.hpp
index a05183f..0dfe2b6 100644
--- a/src/interpreter.hpp
+++ b/src/interpreter.hpp
@@ -25,6 +25,7 @@
     void operator()(const describe_&) const;
     void operator()(const discard_&) const;
     void operator()(const help_&) const;
+    void operator()(const copy_& copy) const;
 
 private:
     template <typename T>