Using nuget v2.1.2 of the ZCRMSDK, I try to call GenerateAccessToken with a self-client grant token. The call throws an exception and when I place a debug proxy on the project I see that it is trying to populate the UserMaiilId (see https://github.com/zoho/zcrm-csharp-sdk/blob/master/ZohoCRM/OAuth/Client/ZohoOAuthClient.cs#L84).