In 2000, Hafner and Stopple proved a conjecture of Zagier which states that the constant term of the automorphic function |∆(x+iy)| 2 i.e., the Lambert series ∞ n=1 τ (n) 2 e −4πny can be expressed in terms of the non-trivial zeros of the Riemann zeta function. In this article, we study an asymptotic expansion of a generalized version of the aforementioned Lambert series associated to Siegel cusp forms.