scroll up and down in a groupchat window throws exception

In a groupchat window, after there appeared a scrollbar, then trying to scroll up and down, the following exception shows up:

bad window path name ".notifycall.f.call.answer"
bad window path name ".notifycall.f.call.answer"
    while executing
"focus [set $wfocus]"
    (procedure "::UI::ScrollSet" line 24)
    invoked from within
"::UI::ScrollSet .jgc1.frall.room.mid.pv.l.ysc {grid .jgc1.frall.room.mid.pv.l.ysc -column 1 -row 0 -sticky ns -padx 2} 0.13522012578616352 0.710691823..."
    (vertical scrolling command executed by text)

doing the same in an ordinary chat window or a message window works fine.