> > I assume IRR=-1.4228295850 -> -2.8421709430404e-14 wouldn't qualify as
> > a solution?
>
> Ah... nice catch! :-)

In the meantime I read HR's post that the function isn't defined for
-100%. I'd assume it isn't defined for any values < -0.999999... As it
was previously stated, negative values probably are difficult to
interpret anyway. It seems in practice people use certain deprecation
rules in order to exclude such values.