gitk error on a single commit

Pointing gitk (git version 0.99.9l) on commit 5569bf9bbedd63a00780fc5c110e0cfab3aa97b9 gives this error:

can't read "treediffs(5569bf9bbedd63a00780fc5c110e0cfab3aa97b9 
fae22ac9d7b5fd8bbf0fcfb01aab01c27d84912f)": no such element in array
can't read "treediffs(5569bf9bbedd63a00780fc5c110e0cfab3aa97b9 
fae22ac9d7b5fd8bbf0fcfb01aab01c27d84912f)": no such element in array
    while executing
"set files $treediffs([list $diffmergeid $p])"
    (procedure "contmergediff" line 42)
    invoked from within
"contmergediff {}"
    (procedure "mergediff" line 12)
    invoked from within
"mergediff $id"
    (procedure "selectline" line 113)
    invoked from within
"selectline $l 1"
    (procedure "selcanvline" line 16)
    invoked from within
"selcanvline .ctop.top.clist.canv 258 218"
    (command bound to event)

...and I think this doesn't happen with any other commit in git's history (so far). What could the reason be?