Reason

I common mistake is to use a nonlinear variable name in the 'vars', 'vals' inputs. If a non-linear variable is supplied MOOSE should provide a message explaining that it can't be done.

Design

Check the 'vals' parameter against the known nonlinear va

add_element!(mesh, elid, eltype, connectivity)
Add an element into the mesh. ´elid´ is the element id, ´eltype´ is the type of the element and ´connectivity´ is the connectivity of the element.

At least eltype needs to be defined more precisely. I guess it's a symbol :Seg2, :Tri3 and so on.