calling CRM_Core_BAO_CustomValue::updateValue($contactId, $cfId, $value) results in a fatal error

calling the static method CRM_Core_BAO_CustomValue::updateValue($contactId, $cfId, $value) in crm/core/bao/CustomValue.php results in a fatal error.

the error message is:
Fatal error: NO $__table SPECIFIED in class definition in C:\wamp\www\drupal-5.7\sites\all\modules\civicrm\packages\DB\DataObject.php on line 407

this error appears to be generated by the following code which is present at the start of the method definition:

$customValue =& new CRM_Core_BAO_CustomValue();

$customValue->custom_field_id = $cfId;
$customValue->entity_table = 'civicrm_contact';
$customValue->entity_id = $contactId;

$customValue->find(true);

i don't yet know civicrm well enough to investigate further.