Error: double line smoothness (can't use non-numeric string as operand of "-")

[sander]

1) Draw a line using the brush tool
2) RMB click and select "Line Smoothness"
3) RMB click and select "Line Smoothness", you will now see next error:

can't use non-numeric string as operand of "-"
can't use non-numeric string as operand of "-"
    while executing
"expr 1 - $smooth"
    (procedure "::CanvasUtils::ItemSmooth" line 9)
    invoked from within
"::CanvasUtils::ItemSmooth .wb1.fmain.fc.can 71"
    (menu invoke)

[Mats (matsben)]

This was a result of a 8.5 behaviour.