[Tugindia] Diagram message

[Naveed Shams]

I want to make the diagram attached with the email message. I am using the following code in LaTeX:
\begin{displaymath}
\xymatrix{ \wtu \[r]^{\lambda} \ar[d]_{\pi} & \wtu' \ar[d]^{\pi'} \\
\wtu/\Ga \ar[d]_{\sim} & \wtu'/\Ga' \ar[d]^{\sim}\\ U \[r]& U'}
\end{displaymath}
But I am getting the following error message:
***********************
! Undefined control sequence.
<argument> ...\COORD@ =\xymatrixCOORD@ \setbox \z
@ \hbox {\frozen @everymat...
l.45 \xy@{\save }{\save@ }
%
The control sequence at the end of the top line
of your error message was never \def'ed. 
************************
The problem is that I have used it in another environment successfully and the parameters are pretty much the same in both environments. 
Any ideas?
Thanks,
Naveed