diff --git a/include/loki/flex/flex_string_shell.h b/include/loki/flex/flex_string_shell.h
index 2af3f2c..3e967ff 100644
--- a/include/loki/flex/flex_string_shell.h
+++ b/include/loki/flex/flex_string_shell.h
@@ -186,6 +186,7 @@ class flex_string : private Storage
 #else
         Invariant(const flex_string&) {} 
 #endif
+    private:
     Invariant& operator=(const Invariant&);
     };