Index: src/wp-includes/user.php
===================================================================
--- src/wp-includes/user.php	(revision 52649)
+++ src/wp-includes/user.php	(working copy)
@@ -1995,6 +1995,10 @@
 	 */
 	$user_nicename = apply_filters( 'pre_user_nicename', $user_nicename );
 
+	if (mb_strlen( $user_nicename ) > 50) {
+		$user_nicename = mb_substr( $user_nicename, 0, 50 );
+	}
+
 	$user_nicename_check = $wpdb->get_var( $wpdb->prepare( "SELECT ID FROM $wpdb->users WHERE user_nicename = %s AND user_login != %s LIMIT 1", $user_nicename, $user_login ) );
 
 	if ( $user_nicename_check ) {