diff --git a/lemon/concepts/path.h b/lemon/concepts/path.h
--- a/lemon/concepts/path.h
+++ b/lemon/concepts/path.h
@@ -66,7 +66,10 @@
 
       /// \brief Template assigment
       template <typename CPath>
-      Path& operator=(const CPath& cpath) {}
+      Path& operator=(const CPath& cpath) {
+        ignore_unused_variable_warning(cpath);
+        return *this;
+      }
 
       /// Length of the path ie. the number of arcs in the path.
       int length() const { return 0;}