I have a model called Article. The primary key is 'id'

When I do this in my controller:
$article = new Article;
$row = $article->find(37, 'id');

If get this error:
Fatal error: Uncaught exception 'Zend_Db_Table_Exception' with message 'Too many columns for the primary key'


Does anyone know what the problem is with this? Thanks for any help guys.