Batch Shipment creation: "Invalid JSON received from the Shippo API"

[ryandlittle]

Client is throwing ShippoAPIError for batch shipment creation with batch sizes equal or greater than 500. Small batches (1-200) return a valid JSON response.

{
    "type": "ShippoAPIError",
    "message": "Invalid JSON received from the Shippo API"
}

Manually testing large batches shipment requests (>=500) with Postman seem to return valid JSON and I'm able to use the resulting batch shipment.