diff --git a/source/lib/readline.c b/source/lib/readline.c
index 2475017adf0..543e4c3349c 100644
--- a/source/lib/readline.c
+++ b/source/lib/readline.c
@@ -73,7 +73,13 @@ char *smb_readline(char *prompt, void (*callback)(void),
                                                 completion_fn);
 
 	if (completion_fn) {
-		rl_attempted_completion_function = completion_fn;
+		/* The cast is here because the callback prototype has
+		   changed slightly between different versions of
+		   Readline.  The same function works in all of them
+		   to date, but we get compiler warnings without the
+		   cast. */
+		rl_attempted_completion_function =
+			(rl_completion_func_t *) completion_fn;
 	}
 
 	if (callback) rl_event_hook = (Function *)callback;