Grading optimization - pond design - berm slope

[G_-K_]

Hi guys,

I'm having an issue here, probably something stupid, but I can't change the berm inside slope (Pente intérieure (H:1V) on the pic below) the way I want it.

I want a 1.5:1H slope, but I either I type 1.5:1 or 1,5:1, all I get is NaN:1 or ∞:1 

What am I missing here ?

Thanks for your help.