Index: src/wp-admin/includes/class-wp-ms-users-list-table.php
===================================================================
--- src/wp-admin/includes/class-wp-ms-users-list-table.php	(revision 40579)
+++ src/wp-admin/includes/class-wp-ms-users-list-table.php	(working copy)
@@ -258,7 +258,11 @@
 	 * @param WP_User $user The current WP_User object.
 	 */
 	public function column_name( $user ) {
-		echo "$user->first_name $user->last_name";
+		if ( ! empty( $user->first_name ) || ! empty( $user->last_name ) ) {
+			echo "$user->first_name $user->last_name";
+		} else {
+			echo _x('Unknown');
+		}
 	}
 
 	/**