diff -urpN john-1.7.8.orig//src/john.c john-1.7.8/src/john.c
--- john-1.7.8.orig//src/john.c	2011-02-27 12:31:36.000000000 +0000
+++ john-1.7.8/src/john.c	2011-08-23 00:01:26.000000000 +0000
@@ -129,8 +129,23 @@ static void john_load(void)
 
 	umask(077);
 
-	if (options.flags & FLG_EXTERNAL_CHK)
+	if (options.flags & FLG_EXTERNAL_CHK) {
 		ext_init(options.external);
+		if (options.flags & (FLG_BATCH_CHK | FLG_SINGLE_CHK |
+			    FLG_WORDLIST_CHK | FLG_INC_CHK | FLG_MAKECHR_CHK))
+#ifdef FLG_MKV_CHK
+		if (options.flags & FLG_MKV_CHK)
+#endif
+		if (!f_filter) {
+			log_event("! No filter() function defined");
+#ifdef HAVE_MPI
+			if (mpi_id == 0)
+#endif
+			fprintf(stderr, "No filter() for external mode: %s\n",
+				ext_mode);
+			error();
+		}
+	}
 
 	if (options.flags & FLG_MAKECHR_CHK) {
 		options.loader.flags |= DB_CRACKED;