Use of amsmath binom causes missing number if breqn is used

I was hitting the following mysterious error:

./xx.tex:7: Missing number, treated as zero.
<to be read again>
                   relax
l.7 binom{n}{k}

and I tracked it down to the breqn and flexisym packages vs. amsmath. Here is a MWE:

documentclass[letterpaper]{article}
usepackage{amsmath}
usepackage{flexisym}
usepackage{breqn}
begin{document}
begin{equation}
binom{n}{k}
end{equation}
end{document}

With either the flexisym or breqn usepackage’s left in, I hit the error above, but with neither left in, binom works fine.

I’m running texlive-2019.20200218-1, breqn 0.98h from WSL2 Ubuntu 20.04.1 LTS.

Is this a known problem? Are there any work arounds?