Comment 3 for bug 1986544

This is a result of line 59 of the post install script failing.

line 59: invoke-rc.d timidity stop

code section:

# make sure we really stop, because packaging system doesn't
# understand what we're trying to do with migration to timidity-daemon
[ -f "/etc/init.d/timidity" ] && if which invoke-rc.d >/dev/null 2>&1; then
  invoke-rc.d timidity stop
else
  /etc/init.d/timidity stop
fi

as a workaround I prefaced the line with echo and the install completed successfully.
another check is needed to see if timidity is running and this stop is necessary.