Am trying to construct a simple update query in my model

class Model_DbTable_Account extends Zend_Db_Table_Abstract
{
    protected $_name = 'accounts';
    public function activateaccount($activationcode)
    {

        $data = array(              
                'accounts_status' => 'active',              
        );

        $this->update($data, 'accounts_activationkey = ' . $activationcode);        
    }

However I get an

    SQLSTATE[42S22]: Column not found: 1054 Unknown column 'my activation code value' 
in 'where clause' 

error.

What am I missing in Zend_Table update construct?