negative paint skin weights value

I write simple tool that help me to skin and I need access to change paint skin weight strength value.
It work when value is positive, however I also need to set negative value in order to add and remove skin values from vertices.

When I use thePainterInterface.maxStr=-1 I got

"Parameter value out of range: < 0 or > 1e+06: -1"

when I use:

I got ok in listner but skin strength remain unchanged.

Do anybody know how to set negative skin value by maxscript like you can do manually in Painter options max.strength val to below zero.