diff --git a/appl/kx/krb4.c b/appl/kx/krb4.c
index a34cca41e..9423fae41 100644
--- a/appl/kx/krb4.c
+++ b/appl/kx/krb4.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1995 - 1999 Kungliga Tekniska Högskolan
+ * Copyright (c) 1995 - 2000 Kungliga Tekniska Högskolan
  * (Royal Institute of Technology, Stockholm, Sweden).
  * All rights reserved.
  * 
@@ -218,6 +218,11 @@ krb4_copy_encrypted (kx_context *kc,
 	fd_set fdset;
 	int ret;
 
+	if (fd1 >= FD_SETSIZE || fd2 >= FD_SETSIZE) {
+	    warnx ("fd too large");
+	    return 1;
+	}
+
 	FD_ZERO(&fdset);
 	FD_SET(fd1, &fdset);
 	FD_SET(fd2, &fdset);