Wouldn't this need to know what I actually did instead?

When I use it, it tells me that the form was incomplete. I tried to figure out how much I regret having bought 0 bitcoin so far.

Perhaps needs a better error message? I think it actually doesn't like the zero, but prevents it doesn't like incomplete data. Zero is also a number.

> CAUSALITY_ERROR: You cannot SELL before you BUY.

https://en.wikipedia.org/wiki/Short_(finance)

Needs at least a better error message that short sells aren't supported, instead of trying to be too clever by half about causality.