Yes, thanks to Shawn's super useful quick start guide, that was the first thing I did :)
I'm quite sure it's the integer vs fractional number thing, I can reproduce it any time I try to submit a non-integer number and the call works perfectly if it's an integer. I've done a very easy fix: I round values to two decimals then multiply by 100 and parse the result as an integer to be 1000% safe :)