0x api v3 -> {"reason":"Could not find field 'makerAssetAmount' while converting string fields to BigNumber."}

Question

I am trying to submit an order using the endpoint https://api.0x.org/sra/v3/order.

I get as response

{"reason":"Could not find field 'makerAssetAmount' while converting string fields to BigNumber."}

even if makerAssetAmount exists in the JSON data I pass.

Indeed, if I remove it before submitting the request I get

{"code":100,"reason":"Validation Failed","validationErrors":[{"field":"makerAssetAmount","code":1000,"reason":"requires property "makerAssetAmount""}]}