diff --git a/py/binary.c b/py/binary.c
index 8b5c05ab3e..ee00c4e348 100644
--- a/py/binary.c
+++ b/py/binary.c
@@ -291,6 +291,10 @@ void mp_binary_set_val(char struct_type, char val_type, mp_obj_t val_in, byte **
             #endif
             {
                 val = mp_obj_get_int(val_in);
+                // sign extend if needed
+                if (BYTES_PER_WORD < 8 && size > sizeof(val) && is_signed(val_type) && (mp_int_t)val < 0) {
+                    memset(p + sizeof(val), 0xff, size - sizeof(val));
+                }
             }
     }
 
diff --git a/py/mpz.c b/py/mpz.c
index 3c20023bc5..3bd3090411 100644
--- a/py/mpz.c
+++ b/py/mpz.c
@@ -1441,8 +1441,8 @@ void mpz_as_bytes(const mpz_t *z, bool big_endian, mp_uint_t len, byte *buf) {
         for (; bits >= 8; bits -= 8, d >>= 8) {
             mpz_dig_t val = d;
             if (z->neg) {
-                d = (~d & 0xff) + carry;
-                carry = d >> 8;
+                val = (~val & 0xff) + carry;
+                carry = val >> 8;
             }
             if (big_endian) {
                 *--b = val;