diff --git src/wp-includes/ms-functions.php src/wp-includes/ms-functions.php
index 09b48be..f60cd95 100644
--- src/wp-includes/ms-functions.php
+++ src/wp-includes/ms-functions.php
@@ -715,6 +715,7 @@ function wpmu_validate_blog_signup( $blogname, $blog_title, $user = '' ) {
 		$errors->add( 'blogname', __( 'Sorry, that site already exists!' ) );
 	}
 
+	// Check if the blogname matches an existing user's login name.
 	if ( username_exists( $blogname ) ) {
 		if ( ! is_object( $user ) || ( is_object( $user ) && ( $user->user_login != $blogname ) ) ) {
 			$errors->add( 'blogname', __( 'Sorry, that site is reserved!' ) );