diff --git a/Include/objimpl.h b/Include/objimpl.h
index 4716517bb93..e6f9929f3d8 100644
--- a/Include/objimpl.h
+++ b/Include/objimpl.h
@@ -28,4 +28,4 @@ extern int StopPrint; /* Set when printing is interrupted */
 /* Malloc interface */
 #include "malloc.h"
 
-extern char *strdup PROTO((char *));
+extern char *strdup PROTO((const char *));