diff --git a/Include/objimpl.h b/Include/objimpl.h
index 4716517..e6f9929 100644
--- a/Include/objimpl.h
+++ b/Include/objimpl.h
@@ -28,4 +28,4 @@
 /* Malloc interface */
 #include "malloc.h"
 
-extern char *strdup PROTO((char *));
+extern char *strdup PROTO((const char *));