diff --git a/src/scripting/lua/core.c b/src/scripting/lua/core.c
index 830676b5..fcd71764 100644
--- a/src/scripting/lua/core.c
+++ b/src/scripting/lua/core.c
@@ -71,7 +71,13 @@ static void handle_ref(LS, struct session *ses, int func_ref,
 static int
 l_alert(LS)
 {
-	alert_lua_error((unsigned char *) lua_tostring(S, 1));
+	unsigned char *msg = (unsigned char *) lua_tostring(S, 1);
+
+	/* Don't crash if a script calls e.g. error(nil) or error(error).  */
+	if (msg == NULL)
+		msg = "(cannot convert the error message to a string)";
+
+	alert_lua_error(msg);
 	return 0;
 }