I am using Partner WSDL to perform Upsert in my .NET application. I am splitting the records in batches of 200 and using the wrapper classes to compress the request and response using gzip.

I am using FieldsToNull property to mark the fields that are null in each of the sObjects in my Upsert array. And I am also providing the Id property of the records where available. Of course, new records will not have the Id property.

However, I am getting the "Internal Server Errror" response when trying to upsert. I have never seen this error before and I don't see any other error codes that tells me precisely what's wrong with what I am trying to do.

Any help is appreciated.