diff --git a/library/nist_kw.c b/library/nist_kw.c
index 1b4078e..d5b2e27 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 = mbedtls_ct_int_if(mbedtls_ct_uint_gt(padlen, 7),
-                                MBEDTLS_ERR_CIPHER_AUTH_FAILED, ret);
+        ret = -(int) mbedtls_ct_uint_if(mbedtls_ct_uint_gt(padlen, 7),
+                                        -MBEDTLS_ERR_CIPHER_AUTH_FAILED, -ret);
         padlen &= 7;
 
         /* Check padding in "constant-time" */