protected function _createNullTerm( $name )
	{
		switch( $this->getOperator() )
		{
			case '==':
				return $name . ' === null';
			case '!=':
				return $name . ' !== null';
			default:
				throw new MW_Common_Exception( sprintf( 'null value not allowed for operator "%1$s"', $this->getOperator() ) );
		}
	}