One of the character's in my campaign is in debt and, as a joke, I put in negative gold, which caused the following error.  I think you may want to validate that input so its not ever negative.

Action: character.CURRENCIES_SET

Payload: {"cp":0,"sp":0,"gp":0,"ep":0,"pp":-1}

Error: ActionApiError: Invalid JSON response, data object is a string at new t (https://www.dndbeyond.com/Content/1-0-538-0/React/CharacterTools/dist/characterSheet.bundle.min.js:23:48429) at Object.r [as validateResponse] (https://www.dndbeyond.com/Content/1-0-538-0/React/CharacterTools/dist/characterSheet.bundle.min.js:23:47028) at https://www.dndbeyond.com/Content/1-0-538-0/React/CharacterTools/dist/characterSheet.bundle.min.js:50:386409 at r (https://www.dndbeyond.com/Content/1-0-538-0/React/CharacterTools/dist/characterSheet.bundle.min.js:23:248603) at Generator._invoke (https://www.dndbeyond.com/Content/1-0-538-0/React/CharacterTools/dist/characterSheet.bundle.min.js:23:249723) at Generator.e.(anonymous function) [as next] (https://www.dndbeyond.com/Content/1-0-538-0/React/CharacterTools/dist/characterSheet.bundle.min.js:23:248782) at h (https://www.dndbeyond.com/Content/1-0-538-0/React/CharacterTools/dist/characterSheet.bundle.min.js:23:123723) at n (https://www.dndbeyond.com/Content/1-0-538-0/React/CharacterTools/dist/characterSheet.bundle.min.js:23:124394)

I just got the same thing, for the same reason (character in debt). In case it helps: