  Consider the divisor sum $\sum_{n\leq N}\tau(n^2+2bn+c)$ for integers $b$ and
$c$. We extract an asymptotic formula for the average divisor sum in a
convenient form, and provide an explicit upper bound for this sum with the
correct main term. As an application we give an improvement of the maximal
possible number of $D(-1)$-quadruples.
