diff --git a/library/nist_kw.c b/library/nist_kw.c
index 5054ca2..3fff2b7 100644
--- a/library/nist_kw.c
+++ b/library/nist_kw.c
@@ -169,7 +169,7 @@
     size_t i = 0;
     for( i = 0; i < sizeof( t ); i++ )
     {
-        A[i] ^= ( t >> ( ( sizeof( t ) - 1 - i ) * 8 ) ) & 0xff;
+        A[i] ^= BYTE_0( t >> ( ( sizeof( t ) - 1 - i ) * 8 ) );
     }
 }