diff --git a/library/ssl_tls12_server.c b/library/ssl_tls12_server.c
index 53a9ce2..5bee188 100644
--- a/library/ssl_tls12_server.c
+++ b/library/ssl_tls12_server.c
@@ -2703,8 +2703,7 @@
                              PSA_KEY_TYPE_ECC_KEY_PAIR(ssl->handshake->xxdh_psa_type));
             psa_set_key_bits(&key_attributes, ssl->handshake->xxdh_psa_bits);
 
-            key_len = PSA_BITS_TO_BYTES(key->grp.pbits);
-            ret = mbedtls_ecp_write_key(key, buf, key_len);
+            ret = mbedtls_ecp_write_key_ext(key, &key_len, buf, sizeof(buf));
             if (ret != 0) {
                 mbedtls_platform_zeroize(buf, sizeof(buf));
                 break;