diff --git a/core/modules/node/content_types.inc b/core/modules/node/content_types.inc
index 8927940..041ffd0 100644
--- a/core/modules/node/content_types.inc
+++ b/core/modules/node/content_types.inc
@@ -128,7 +128,8 @@ function node_type_form($form, &$form_state, $type = NULL) {
   $form['type'] = array(
     '#type' => 'machine_name',
     '#field_prefix' => 'custom_',
-    '#default_value' => $type->type,
+    // Remove the 'custom_' prefix from the default value.
+    '#default_value' => preg_replace('/^custom_/', '', $type->type),
     // 32 characters minus the 'custom_' prefix.
     '#maxlength' => 25,
     '#disabled' => $type->locked,
@@ -308,12 +309,8 @@ function _node_characters($length) {
  */
 function node_type_form_validate($form, &$form_state) {
   $type = new stdClass();
-  $type->type = trim($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);
-  }
+  $type->type = 'custom_' . trim($form_state['values']['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