Script for limit order with fraction doesn't work

[wilko]

I can get a simple limit order script to work that enters a limit price to buy at the current Ask price less 1 point for the FTSE 100. I also wanted to have a script to enter a limit of Ask price minus a 0.5 but this does not work for me. No fractional scripts seem to work, only whole numbers.
So I work be very grateful to anyone who can help me out.
Thanks