Could it be that you're using "redeemscript" instead of "redeemScript", ie with a small "s" instead of a capital one?
Yep, what dserrano5 said. The error should point you in the right direction: "Missing redeemScript".
I'm an idiot. Thanks.
It pointed me at the right direction, but I didn't notice the case discrepancy, and since this datum was included in previous calls I assumed this kind of an error would have caused problems earlier.
In case you were wondering why it didn't trigger on previous calls, the redeemScript isn't needed until signing. You can stuff arbitrarily named members into that object without a problem. Those functions only check to make sure they have the parts they need, they don't reject extra stuff, they just ignore it.