Error while selecting from combobox

This is random error, could not be reproduced yet.
It happend while the user selected a value from a ttk::combobox.
There is a trace to the combobox variable, added with: trace add
variable $varname write $callback
But the error does not seem to have anything with the callback function.
It's a rather internal issue.
Any ideas?
Thanks
Alex
*** END OF ERROR MESSAGE ***} -errorlog 1}
1: {::bgerrorhandler
{Index 2 out of range}
{-code 1
 -level 0
-errorstack
    {INNER {invokeStk1
vpan.main.workframe.detailsframe.sw.sf.mf.cf.asmpar.solve.cb3 current
2}
    CALL {SelectEntry
vpan.main.workframe.detailsframe.sw.sf.mf.cf.asmpar.solve.cb3 2}
    CALL {LBSelect
vpan.main.workframe.detailsframe.sw.sf.mf.cf.asmpar.solve.cb3.popdown.f.l}
    CALL {ttk::combobox::LBSelected
vpan.main.workframe.detailsframe.sw.sf.mf.cf.asmpar.solve.cb3.popdown.f.l}
}
-errorcode
    {TTK COMBOBOX IDX_RANGE}
-errorinfo
    {Index 2 out of range
    while executing
"$cb current $index"
    (procedure "SelectEntry" line 2)
    invoked from within
"SelectEntry $cb [lindex $selection 0]"
    (procedure "LBSelect" line 5)
    invoked from within
"LBSelect $lb"
    (procedure "ttk::combobox::LBSelected" line 3)
    invoked from within
"ttk::combobox::LBSelected
vpan.main.workframe.detailsframe.sw.sf.mf.cf.asmpar.solve.cb3.popdown.f.l
"
    (command bound to event)} -errorline 1}}
0: {::meshparts::showstack file2a8350dd0}
--