diff --git a/core/modules/node/content_types.inc b/core/modules/node/content_types.inc
index 067e118..8927940 100644
--- a/core/modules/node/content_types.inc
+++ b/core/modules/node/content_types.inc
@@ -308,8 +308,12 @@ function _node_characters($length) {
  */
 function node_type_form_validate($form, &$form_state) {
   $type = new stdClass();
-  $type->type = 'custom_' . trim($form_state['values']['type']);
-  form_set_value($form['type'], $type->type, $form_state);
+  $type->type = trim($form_state['values']['type']);
+  // If this is a new type, append 'custom_' to the name.
+  if (empty($form_state['values']['old_type'])) {
+    $type->type = 'custom_' . $type->type;
+    form_set_value($form['type'], $type->type, $form_state);
+  }
   $type->name = trim($form_state['values']['name']);
 
   // Work out what the type was before the user submitted this form