data.input doesnot accept value X and B on first positon

I have a script where I use data.input to request data from the user.

data.input("9 4 2 16 U L='\99\88\77' ", inv.item , "")

For some reason the value X or B are not accepted as the first character.

When I use the option T='ABCDEFGHIJKLMNOPQRSTUVWXYZ', value B is accepted, but still not X.

Any ideas ?