  Calabi and Cheng-Yau's Bernstein-type theorem asserts that an entire zero
mean curvature graph in Lorentz-Minkowski $(n+1)$-space $\boldsymbol R^{n+1}_1$
which admits only space-like points is a hyperplane. Recently, the third and
fourth authors proved a line theorem for hypersurfaces at their degenerate
light-like points. Using this, we give an improvement of the Bernstein-type
theorem, and we show that an entire zero mean curvature graph in $\boldsymbol
R^{n+1}_1$ consisting only of space-like or light-like points is a hyperplane.
This is a generalization of the first, third and fourth authors' previous
result for $n=2$.
