PLZ HELP - null pointer deference error while opening device

while running a code to open a usb-serial device as
fd = open ("/dev/ttyUSB0",O_RDONLY);

it shows seg fault...with error unable to handle kernel null pointer dereference.

I am running Montavista Linux 2.4 on a palmtop.
Any kinda help will be appreciated.

Thanks
Prateek.

when go two open site, it comes up can not find "(null)"