N.x No operator .x for type <Pl>

Can someone help me look at the following error.
I have a boundary integral:

+int1d(th,5)(-(N.x*lx-N.y*ly)*uh-(N.y*lx+N.x*ly)*vh)

where (lx, ly) is a local force, and I want to transform it to the global coordinate system.

The compiler complains:

N.x No operator .x for type

Thanks for help in advance!

Yongxing.

You use an other variable call N in you script.