[Xy-pic] bent arrow problem

[Max Forester]

Hello,

Here is a simple commutative diagram which is causing me trouble: 

\xymatrix{
A \ar[r] & B \ar[r] & C \\
D \ar[u] \ar[r] & E \ar[u] \ar `^u[r] [ur] & \\
F \ar[u] \ar `^u[r] [ur] & & }

There are two bent arrows that should be identical. The first comes out
fine but the second generates the message 

     Xy-pic Warning: division by 0 in \intersect@, treated as 0 

and has a sharp corner rather than a bend. If I omit the first instance
then the second works fine.  

Is this a bug? Any help would be appreciated. 

regards,
max