diff --git a/appl/telnet/libtelnet/kerberos5.c b/appl/telnet/libtelnet/kerberos5.c
index 23b616a80..1e8f97810 100644
--- a/appl/telnet/libtelnet/kerberos5.c
+++ b/appl/telnet/libtelnet/kerberos5.c
@@ -425,6 +425,15 @@ kerberos5_is(Authenticator *ap, unsigned char *data, int cnt)
 	    return;
 	}
 
+	if (key_block == NULL) {
+	    Data(ap, KRB_REJECT, "no subkey received", -1);
+	    auth_finished(ap, AUTH_REJECT);
+	    if (auth_debug_mode)
+		printf("Kerberos V5: "
+		       "krb5_auth_con_getremotesubkey returned NULL key\r\n");
+	    return;
+	}
+
 	if ((ap->way & AUTH_HOW_MASK) == AUTH_HOW_MUTUAL) {
 	    ret = krb5_mk_rep(context, auth_context, &outbuf);
 	    if (ret) {