diff --git a/library/ccm.c b/library/ccm.c
index 1e88f90..0a90461 100644
--- a/library/ccm.c
+++ b/library/ccm.c
@@ -171,7 +171,7 @@
     ctx->y[0] |= ctx->q - 1;
 
     for( i = 0, len_left = ctx->plaintext_len; i < ctx->q; i++, len_left >>= 8 )
-        ctx->y[15-i] = (unsigned char)( len_left & 0xFF );
+        ctx->y[15-i] = MBEDTLS_BYTE_0( len_left );
 
     if( len_left > 0 )
     {