serial question - port index?

really quick serial question - the helpfile for the serial object
says that the "port" message can take either a letter (a,b,c,d...) or
the port name as a symbol - but it doesn't complain when
(accidentally) I hooked up an ubumenu and used a numeric index
(0,1,2,3...). Is this hidden feature, or is it just ignoring the
number and using the first serial port anyway (port a)?

Thanks,
Evan