set baud speed for tty

How do I set the baud speed for a specific tty (in this case a serial port)? I tried using stty -F /dev/tty.iap ispeed 19200 But get the error Invalid argument for every speed I try (2400, 4800, etc.) except 9600 (the default). I can successfully change the speed in a C program using cfsetspeed. Do I need to make a change somewhere else, like the ttys file?