Call to a member function free() on a non-object

I just upgraded your good class.
Found one error:

public function query( $query )
{
$full_query = $this->link->query( $query );
if( $this->link->error )
{
$this->log_db_errors( $this->link->error, $query, 3, "query" );
$full_query->free();
return false;
}
else
{
$full_query->free(); ---------Error line
return true;
}
}
I'm not using this function outside of class.
Do you may be know what is the problem?
Thank you

Hi Miodrag!

I've fixed the error and updated the class