  In 2000, Hafner and Stopple proved a conjecture of Zagier which states that
the constant term of the automorphic function $|\Delta(x+iy)|^2$ i.e., the
Lambert series $\sum_{n=1}^\infty \tau(n)^2 e^{-4 \pi n y}$ can be expressed in
terms of the non-trivial zeros of the Riemann zeta function. In this article,
we study a certain Lambert series associated to Siegel cusp forms and observe a
similar phenomenon.
