  Let $D$ be a bounded convex domain in $\mathbb{C}$ with a regular analytic
boundary. Suppose that the numerical range $W(A)$ of a bounded linear operator
$A$ is contained in $\overline{D}$. If $\overline{W(A)}$ intersects the
boundary $\partial D$ at infinitely many points while the essential numerical
range $W_\text{ess}(A)$ does not intersect $\partial D$, then $W(A) =
\overline{D}$. This generalizes some infinite dimensional analogues of a result
of Anderson.
