Complement of a plane with a parameter

In ${\mathbb{R}}^3$ consider the plane $U:x+y-z=0$ and the line $W_k=⟨(1,k,1)⟩$, with $k\in \mathbb{R}$. Determine for which values of $k$ we have

$${\mathbb{R}}^3=U\oplus W_k.$$