Error using form API on AJAX submit

Question

I am writing a form using the Drupal 7 form API. On submitting the form (via AJAX), the following error is displayed:

An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: /en/system/ajax StatusText: Internal Server Error ResponseText: Recoverable fatal error: Argument 1 passed to drupal_array_nested_key_exists() must be an array, null given, called in /drupal7/includes/form.inc on line 1986 and defined in drupal_array_nested_key_exists() (line 6296 of /drupal7/includes/common.inc).

I have two buttons on the form (copy & submit); one is an AJAX call which does some stuff, and the other is used to save the entered data.

This is the code I'm using:

function foo_form ($form, &$form_state, $foo) {
  $form['#foo'] = $foo;
  $form_state['values'] = $foo;

  $form['#prefix'] = '<div id="foo-form-wrapper">';
  $form['#suffix'] = '</div>';

  $form['copy'] = array(
    '#type' => 'submit',
    '#value' => t('Copy foo'),
    '#limit_validation_errors' => array(),
    '#submit' => array('foo_form_submit'),
    '#ajax' => array(
      'wrapper' => 'foo-form-wrapper',
      'callback' => 'foo_form_callback',
    ),
  );

  $form['name'] = array(
    '#type' => 'textfield',
    '#title' => t('Name'),
    '#required' => TRUE,
    '#maxlength' => 255,
    '#default_value' => (!isset($form_state['values']['name']) ? $foo->name : $form_state['values']['name']),
  );

  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Save'),
  );

  return $form;
}

function foo_form_callback($form, $form_state) {
  return $form;
}

function foo_form_submit ($form, &$form_state) {
  // Did we use the AJAX Call?
  // @todo: can we check this some other way?
  if (!isset($form_state['triggering_element']['#ajax'])) {

    // We didn't, save the entered values

    $form_state['rebuild'] = false;
    return;
  } else {

    // We did, do some behind the scenes stuff...

    $form_state['rebuild'] = true;
  }
}

Does anyone have an idea of what could be wrong?

Answer 1

This line is wrong:

$form_state['values'] = $foo;

No idea what you are trying to do here, but you must not mess with $form_state['values'] directly.

And what you are trying to do with the #default_value shouldn't be necessary either. Form API should automatically re-use an already entered value, without you having to set it as the default_value.