DPD - Cannot deserialize the current JSON object

The following error (An Error Occurred when trying to Create Shipment - ErrorMessage: Cannot deserialize the current JSON object (e.g. {"name":"value"}) into type 'System.Collections.Generic.List`1[Toolkit.ExternalAPI.Couriers.Geopost.DataClasses.API.Shipping.Result.Error]' because the type requires a JSON array (e.g. [1,2,3]) to deserialize correctly. To fix this error either change the JSON to a JSON array (e.g. [1,2,3]) or change the deserialized type so that it is a normal .NET type (e.g. not a primitive type like integer, not a collection type like an array or List) that can be deserialized from a JSON object. JsonObjectAttribute can also be added to the type to force it to deserialize from a JSON object. Path 'error.errorCode', line 1, position 22.) from DPD is caused when there is a missing or invalid UKIMS Number entered on the Client details page.

Please check the UKIMS Number entered within the Customs Information tab on the Client details page. This is a change made by DPD to ship with them.