riskdist of val.prob couldn't be set to FALSE and remove risk distribution in plot

[EscapeReality846089495]

In document, riskdist of val.prob could be FALSE to omit risk distribution.

I want to remove segments at bottom of plot, I believe that they are segments of risk distribution. So I tried to set riskdist = FALSE when I called val.prob.

But I got an Error which is raised by match.arg in function val.prob:
Error in match.arg(riskdist) : 'arg'要么是NULL，要么是字符矢量。
This means I couldn't set riskdist=FALSE which is rejected by argument check.
Is there another way to remove segments at bottom of plot? Or I have to modify the source code in val.prob.s?
I think it may be a bug. If it really is a bug, I'm glad to contribute to fix it!