  Let $l\geq 3$ and $F$ be a modular form of weight $l/2-1$ on
$\mathrm{O}(l,2)$ which vanishes only on rational quadratic divisors. We prove
that $F$ has only simple zeros and that $F$ is anti-invariant under every
reflection fixing a quadratic divisor in the zeros of $F$. In particular, $F$
is a reflective modular form. As a corollary, the existence of $F$ leads to
$l\leq 20$ or $l=26$, in which case $F$ equals the Borcherds form on
$\mathrm{II}_{26,2}$. This answers a question posed by Borcherds in 1995.
