lost standart input after su - <user>

All,
hope somebody can help here:

wrote a small shell script running under root on //usr/bin/ksh
no specialisties in the profile.
Then the test script:
-------------------------------------------
echo "Now we start, press any key..."
read
su - test -c "id | wc -l" 2>/tmp/x
echo "DONE!!!"
read

--------------------------------------------
This script hangs after "echo DONE!!!"
I have to kill the shell to proceed. It works with:

-------------------------------------------
echo "Now we start, press any key..."
read
su - test -c "id" 2>/tmp/x
echo "DONE!!!"
read

--------------------------------------------

The pipe seems the problem. I the file /tmp/x I get then "Not a terminal"
the user "test" uses /usr/bin/ksh
Extract from /etc/passwd:
test:xxxxxxxxxx:105:20:,,,:/home/test:/usr/bin/ksh
Also a blank profile.

Can somebody verify / help?

Would be greatly appreceated.

Thanks