If I’m not mistaken this is caused by gformPreRenderSniff being run on the coupon page. isEwayForm function is then run, but as the page isn’t a form; the function errors out as, since commit #0535eb5 (looking on the git), the function is type-hinted and, as php is running strictly typed, it fails attempting to cast null to an integer.
(This is my guess from my limited knowledge of php, and a quick investigation.)