Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision 28021)
+++ wp-includes/user.php	(working copy)
@@ -1543,6 +1543,11 @@
 		$ID = (int) $ID;
 		$update = true;
 		$old_user_data = WP_User::get_data_by( 'id', $ID );
+		
+		if ( false === $old_user_data ) {
+			return new WP_Error( 'updating_non_existing_user', __( 'User does not exist!' ) );
+		}
+		
 	} else {
 		$update = false;
 		// Hash the password