Problem with gitk on cygwin

When trying to use gitk from current cygwin-install, I get an error-dialog and no gitk-window.

Error title: "Error in startup script"
Error message:
--------------------------------
.git
    8 [main] git 1620 tty_list::allocate: No tty allocated while executing
"exec git rev-parse --git-dir"
  (procedure "gitdir" line 6)
  invoked from within
"gitdir"
  invoked from within
"set gitdir [gitdir]"
  (file  "/usr/bin/gitk" line 6270)
--------------------------------

Setting the environment variable GIT_DIR produced a slightly different
error, but didn't help.
The new error message is:
--------------------------------
    6 [main] git 1268 tty_list::allocate: No tty allocated while executing
"close $refd"
  (procedure "readrefs" line 47)
  invoked from within
"readrefs"
  (file "/usr/bin/gitk" line 6325)
--------------------------------

Did I forget to install some cygwin package (and it's only a problem of cygwin dependencies), or does gitk on cygwin have a problem currently?