b20 - There is nothing to pick up there.

[TheRani]

That would be because in b20, one of the new features is autopickup for gems. So as soon as you stepped onto that gem, you automatically picked it up. When you tried to pick it up afterward, there was nothing left to pick up because you had picked it up already.

[Final Master]

I run into a similar problem at random times when it comes to attempting to use stairs.