Getting in a mix-up with COMn

[Richmond]

Having got my USB device connected to a Windows 7 box, and the driver 
installed, and determined that the
device is listed at COM3 I put this into a button:

on mouseUp
    open file "COM3:" for read
    read from file COM3: until end
    put it into fld "DData"
end mouseUp

and I got one of those charmingly seasonal red hotcross buns right next 
to line 3,

which seems funny give that I just copied that from the Documentation . . .

Richmond.