# Why does TEM mode not propagate in single conductor system?

In the Wikipedia article it is said:

"In hollow waveguides (single conductor), TEM waves are not possible, since Maxwell's Equations will give that the electric field must then have zero divergence and zero curl and be equal to zero at boundaries, resulting in a zero field (or, equivalently,$$\nabla^2 \Phi=0$$ with boundary conditions guaranteeing only the trivial solution)."

My question is why is it required that the electric field be zero at the boundary of a single conductor system. I know that the tangential component of the field must be zero but why does the perpendicular component vanish in this case?