diff --git a/README.rst b/README.rst
index b2ca337..4e8c470 100644
--- a/README.rst
+++ b/README.rst
@@ -11,6 +11,11 @@ There is a slight difference in a return code when they fail.
 `urlparse_parse_url()` returns the negative error code
 ``URLPARSE_ERR_PARSE`` if it fails.
 
+A caller needs to call `http_parser_url_init()` before
+`http_parser_parse_url()`.  urlparse does not need a similar function
+because `urlparse_parser_url()` initializes ``urlparse_url`` before
+its use.
+
 `url_parser_parse_url()` historically does not follow any standards
 like RFC 3986.  Here is the allowed characters in each URL component:
 
diff --git a/urlparse.h b/urlparse.h
index e39ba71..8941ba2 100644
--- a/urlparse.h
+++ b/urlparse.h
@@ -154,6 +154,9 @@ typedef struct urlparse_url {
  * the URL as a request target that appears in CONNECT request, that
  * is, consisting of only the host and port number.
  *
+ * This function initializes |u| before its use.  If this function
+ * returns nonzero, |u| might not be initialized.
+ *
  * This function returns 0 if it succeeds, or
  * :macro:`URLPARSE_ERR_PARSE`.
  */