Notice: Undefined property: stdClass::$language in _signup_date_get_node_scheduler()

I've set up several sign-up content types and created a few dozen nodes.
When I visit the admin page at admin/content/signup I get the following messages:

Notice: Undefined property: stdClass::$language in _signup_date_get_node_scheduler() (line 112 of [...]/sites/all/modules/signup/includes/date.inc).
Warning: array_push() [function.array-push]: First argument should be an array in date_formatter_format() (line 450 of [...]/sites/all/modules/date/date.module).

The first message goes away if I change the second line of code in _signup_date_get_node_scheduler as follows:

  if (!empty($node->{$field['field_name']}[isset($node->language)?$node->language:'und'][0]['value'])) {

I don't know if this is related: in this admin listing page, all nodes are showing up as closed for signup, although they are actually all open (and signing up works). Only one of the nodes is in a content that has a date field, which has been set to be used by signup; the other nodes don't have any date field.

Using the latest Drupal 7.2 and Date 7.x-2.0-alpha3.