diff --git a/argument.hpp b/argument.hpp
index 3dd2b9e..6afb762 100644
--- a/argument.hpp
+++ b/argument.hpp
@@ -49,7 +49,7 @@
     };
 
     /** @brief optstring as needed by getopt_long */
-    static inline const char* optionstr = "p:?h";
+    static inline const char* const optionstr = "p:?h";
 };
 
 } // namespace led