gitk - error in git repo on cygwin

Beginning with commit "[66e46f37de3ed3211a8ae0e8fc09c063bc3a1e08] gitk: Store ids in rowrangelist and idrowranges rather than row numbers", gitk run on Cygwin encounters an error looking at the git repository. The error occurs during startup (just run gitk on master), with error:

can't read "commitrow(0,8572)": no such element in array
can't read "commitrow(0,8572)": no such element in array
    while executing
"lappend linenos $commitrow($curview,$rid)"
    (procedure "rowranges" line 14)
    invoked from within
"rowranges $id"
    (procedure "optimize_rows" line 21)
    invoked from within
"optimize_rows $rowoptim 0 [expr {$rowoptim + $nr}]"
    (procedure "layoutmore" line 16)
    invoked from within
"layoutmore $tlimit $allread"
    (procedure "chewcommits" line 9)
    invoked from within
"chewcommits 0"
    ("eval" body line 1)
    invoked from within
"eval $script"
    (procedure "dorunq" line 9)
    invoked from within
"dorunq"
    ("after" script)

This doesn't happen on my Linux box, so this is most likely due to a tcl feature introduced more recently than the tcl used in Cygwin (8.4.1). My guess would be that the more recent tcl is more forgiving of an attempted access to a non-existent element.

Mark