diff --git a/include/mbedtls/x509.h b/include/mbedtls/x509.h
index 597053c..2afe4fa 100644
--- a/include/mbedtls/x509.h
+++ b/include/mbedtls/x509.h
@@ -293,7 +293,7 @@
 int mbedtls_x509_sig_alg_gets( char *buf, size_t size, const mbedtls_x509_buf *sig_oid,
                        mbedtls_pk_type_t pk_alg, mbedtls_md_type_t md_alg,
                        const void *sig_opts );
-int mbedtls_x509_key_size_helper( char *buf, size_t size, const char *name );
+int mbedtls_x509_key_size_helper( char *buf, size_t buf_size, const char *name );
 int mbedtls_x509_string_to_names( mbedtls_asn1_named_data **head, const char *name );
 int mbedtls_x509_set_extension( mbedtls_asn1_named_data **head, const char *oid, size_t oid_len,
                         int critical, const unsigned char *val,