gitk: possible bug in diffcmd

Hello.  

I'm using git-1.5.3.7 on linux.  

When I try get use "Make Patch" from by the context menu and click on "Generate"
an error popup appears which states: 
  "Error creating patch: illegal use of | or &|"
I tracked the problem down to line 5064 in gitk, which belongs to diffcmd: 
  set cmd [concat | git diff-tree -r $flags $ids]  
This line produces something like 
  | git diff-tree -r -p 5441937e2... 7394db226...  
As you see there is a leading pipe symbol which caused the error message.  
Is there some special shell to use with gitk?? Or is it just a BUG?

If I remove the | after concat everything works smooth.

Thanks in advance,

 -- Maik