diff --git a/usr.bin/comm/comm.c b/usr.bin/comm/comm.c
--- a/usr.bin/comm/comm.c
+++ b/usr.bin/comm/comm.c
@@ -89,7 +89,7 @@
 	argc -= optind;
 	argv += optind;
 
-	if (argc != 2)
+	if (argc != 2 || !argv[0] || !argv[1])
 		usage();
 
 	fp1 = file(argv[0]);