Hello! Question about $mdb->close()

I start my work with this:

$mdb = new mdb('MSAccess/'.$sm_filename.'.mdb');
$mdb->open();

Then I do some execute operations (insert and select).
By the end, I do

$mdb->close();

BUT! I get the Fatal error "Operation is not allowed when the object is closed" at this line. What does it mean?