diff --git a/compiler/cpp/src/main.h b/compiler/cpp/src/main.h
index 98ca754..b0af5a7 100644
--- a/compiler/cpp/src/main.h
+++ b/compiler/cpp/src/main.h
@@ -106,7 +106,7 @@
  * If new_form is NULL, old_form is assumed to be a language identifier, such as "cpp"
  * If new_form is not NULL, both arguments are used exactly as given
  */
-void error_unsupported_namespace_decl(char* old_form, char* new_form = NULL);
+void error_unsupported_namespace_decl(const char* old_form, const char* new_form = NULL);
 
 /**
  * Flex utilities