--- a/ml_curses.c
+++ b/ml_curses.c
@@ -74,7 +74,7 @@
     Store_field(ret,2,Val_int(z)); \
     CAMLreturn(ret); }
 #define r_string(f)	\
-  { char *ret=f; \
+  { const char *ret=f; \
     if(ret==NULL) failwith("Null pointer"); \
     CAMLreturn(copy_string(ret)); }