diff --git a/lib/des/des.c b/lib/des/des.c
index e115f936d..43d96142f 100644
--- a/lib/des/des.c
+++ b/lib/des/des.c
@@ -52,6 +52,7 @@ RCSID("$Id$");
 #include <stdlib.h>
 #include <string.h>
 #include <krb5-types.h>
+#include <assert.h>
 
 #include "des.h"
 #include "ui.h"
@@ -514,6 +515,7 @@ DES_cfb64_encrypt(const void *in, void *out,
 
     if (forward_encrypt) {
 	int i = *num;
+	assert(i >= 0);
 
 	while (length > 0) {
 	    if (i == 0)
@@ -535,6 +537,7 @@ DES_cfb64_encrypt(const void *in, void *out,
     } else {
 	int i = *num;
 	unsigned char c;
+	assert(i >= 0);
 
 	while (length > 0) {
 	    if (i == 0) {