Exception while creating termset using SharePoint webservice

Hi, 

I am trying to create a new term set in SharePoint 2013 using a custom web service deployed on SharePoint server. I have written below code to create the term set.

SPSecurity.RunWithElevatedPrivileges(delegate()
                {
    using (Impersonator imp = new Impersonator("Username", "Domain", "Password"))
                    {
                        using (SPSite site = new SPSite("http://server:8002/sites/site/"))
                        {
site.AllowUnsafeUpdates = true;
TaxonomySession session = new TaxonomySession(site);
TermStore termStore = session.TermStores["Managed Metadata Service"];
var termStoreAdmin = termStore.TermStoreAdministrators.Where(obj => obj.PrincipalName.Contains("domain\\username")).FirstOrDefault();

if (termStoreAdmin == null)
termStore.AddTermStoreAdministrator("domain\\username");

Group group = termStore.GetGroup(new Guid(groupGuid));
if (group != null && !string.IsNullOrEmpty(termSetName))
{
TermSet termset = group.TermSets.FirstOrDefault(obj => obj.Name.Equals(termSetName));
if (termset == null)
{
termset = group.CreateTermSet(termSetName);
termSetGuid = termset.Id.ToString();
}
SetupNavTermSet(termset, session, site.OpenWeb());
}
termStore.CommitAll();
}
}
}

In this code I am getting exception while executing group.CreateTermSet(termSetName);line, the error is:

Error    :  Value cannot be null.
Source : Microsoft.SharePoint
Error Details :     at Microsoft.SharePoint.Administration.Claims.SPClaimProviderManager.GetUserIdentifierEncodedClaim(IIdentity identity)
   at Microsoft.SharePoint.Taxonomy.Internal.CommonUtilities.GetCurrentUserName()
   at Microsoft.SharePoint.Taxonomy.TaxonomySession.get_CurrentUserName()
   at Microsoft.SharePoint.Taxonomy.Group.CreateTermSet(String name, Guid newTermSetId, Int32 lcid)
   at Microsoft.SharePoint.Taxonomy.Group.CreateTermSet(String name)
   at SplitVisionMetadataManagement.CustomManageMetaDataWCFService.<>c__DisplayClassc.<CreateTaxonomyTermSet>b__8()

Does anybody got this issue and solution?

Thanks in advance...

---

Ravi Patil