We will tackle a conjecture of S. Seo and A. J. Yee, which says that the series expansion $1/(q,-q^3;q^4)_\infty$ has nonnegative coefficients. Our approach relies on an approximation generally nonmodular infinite product $1/(q^a;q^M)_\infty$, where $M$ is positive integer $a$ any $1,2,\ldots,M$.