Why I "can't delete geometry without causing degenerate primitives"?

Hello!
I’m displacing a Grid SOP with a TOP (a black and white image).
The displacement work fine, but now I should delete all the “not displaced” faces of the grid.
I’m not sure how to do that.
I’m trying with the Delete SO, setting a bounding box around the faces that I should keep, but I get the warning “Can’t delete all selected geometry without causing degenerate primitives”


Why?
Many thanks!

---- EDIT ----
Solved. My fault, I was using the grid as Nurbs instead of polygons