Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision 56244)
+++ wp-includes/user.php	(working copy)
@@ -2122,6 +2122,8 @@
 	// user_login must be between 0 and 60 characters.
 	if ( empty( $user_login ) ) {
 		return new WP_Error( 'empty_user_login', __( 'Cannot create a user with an empty login name.' ) );
+	} if( empty(sanitize_title( $user_login ))){
+		return new WP_Error( 'invalid_user_login', __( 'Please enter valid login name.' ) );
 	} elseif ( mb_strlen( $user_login ) > 60 ) {
 		return new WP_Error( 'user_login_too_long', __( 'Username may not be longer than 60 characters.' ) );
 	}