diff --git a/library/nist_kw.c b/library/nist_kw.c
index b0d4137..bde1775 100644
--- a/library/nist_kw.c
+++ b/library/nist_kw.c
@@ -421,8 +421,8 @@
          * larger than 8, because of the type wrap around.
          */
         padlen = in_len - KW_SEMIBLOCK_LENGTH - Plen;
-        ret = (int) mbedtls_ct_uint_if(mbedtls_ct_uint_gt(padlen, 7),
-                                       MBEDTLS_ERR_CIPHER_AUTH_FAILED, ret);
+        ret = -mbedtls_ct_uint_if(mbedtls_ct_uint_gt(padlen, 7),
+                                  -MBEDTLS_ERR_CIPHER_AUTH_FAILED, -ret);
         padlen &= 7;
 
         /* Check padding in "constant-time" */