diff --git a/src/mfm.hpp b/src/mfm.hpp
index 01f3fdb2..acada219 100644
--- a/src/mfm.hpp
+++ b/src/mfm.hpp
@@ -7,5 +7,5 @@ namespace mfm
   int main(int    argc,
            char **argv);
 
-  void dup(Opts::Dup &dup);
+  void dup(const Opts::Dup &dup);
 }