diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 8f4a4f5..d73c73c 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -251,8 +251,8 @@
         return( MBEDTLS_ERR_X509_FEATURE_UNAVAILABLE );
 
     c = buf + 5;
-    ku[0] = (unsigned char)( key_usage      );
-    ku[1] = (unsigned char)( key_usage >> 8 );
+    ku[0] = MBEDTLS_BYTE_0( key_usage );
+    ku[1] = MBEDTLS_BYTE_1( key_usage );
     ret = mbedtls_asn1_write_named_bitstring( &c, buf, ku, 9 );
 
     if( ret < 0 )