Short trade with notional instead of qty throws error

I am using notional instead of quantities for my ordner submition and get an error when trying to go short on a stock stating that you can not open a short with fractional orders. When using notional quantities, most of the times its fractional depeneding on the dollar value and stock prices. So how to deal with that? Why I can go long but not short with notional market orders? Thats beyond me.