Creating Invoices - invalid JSON error

We have been using the API to create lots of invoices which is done automatically from our finance system. However, it has created them all (about 300) without a problem. But we have one that just comes back saying “Invalid JSON” - I’ve checked the Json and its fine. I don’t really want to post the JSON here because there is private customer information. But given that I’ve run it through a JSON checker - is there anything else that could cause this error?

I’ve checked for anything obvious in the data, any unusual characters etc. Also any unusual fields. The amount is larger than other amounts we have invoiced for - so is there any limit to the size of the field or the amounts, that a JSON validator wouldn’t pick up but Free Agent is giving this error? Any advice would be hugely appreciated.

Thanks!