partition with prescribed weights?

I'm also trying the partitioning with prescribed weights. But it doesn't work for me? I'm using the simple example graph in the manual, as follows:
---------------------
7 11 11
4 5 1 3 2 2 1
2 1 1 3 2 4 1
5 5 3 4 2 2 2 1 2
3 2 1 3 2 6 2 7 5
1 1 1 3 3 6 2
6 5 2 4 2 7 6
2 6 6 4 5
----------------------

And I'm calling METIS_WPartGraphKway(). I printed out the input data before the calling. They look all correct.

-----------------------------------------
xadj[7]=0 3 6 10 14 17 20 22
adjncy[22]=4 2 1 0 2 3 4 3 1 0 1 2 5 6 0 2 5 4 3 6 5 3
vwgt[7]=4 2 5 3 1 6 2
adjwgt[22]=1 2 1 1 2 1 3 2 2 2 1 2 2 5 1 3 2 2 2 6 6 5
wgtflag=3
numflag=0
nparts=2
tpwgts[2]=0.500000 0.500000
-----------------------------------------

But the result is just one single partition, with edgecut=0.

Does this just because my graph is too small to try out this function? Or did I miss something? (Sorry I'm throwing in so many questions all at once....) Thanks!

-Lei