REST XML (APIv3) returns embedded error message, malforming the XML (ref: CRM/Utils/Array.php line 114)

Hi,

I'm trying to use APIv3 to search for a contact based on a custom field. The XML being returned is malformed, it includes a PHP error message referencing the CiviCRM "Array.php" file.

The API call I'm making (although several others generate the same error):

http://HOSTNAME/sites/all/modules/civicrm/extern/rest.php?version=3&entity=Contact&action=get&custom_19=X123456&key=KEY&PHPSESSID=lorem&api_key=ipsum

Specific error message returned before the proper XML:

Invalid argument supplied for foreach() in <b>/public_html/sites/all/modules/civicrm/CRM/Utils/Array.php</b> on line <b>114</b>.

The correct XML is being included after the wrongly-included error messages so I can parse it with my scripts but I can't test in the browser. Any debugging advice?

Thanks!