couchbase nullreference exception

System.NullReferenceException was unhandled
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=Couchbase
  StackTrace:
       at Couchbase.CouchbasePool.Couchbase.ICouchbaseServerPool.GetVBucket(String key)
       at Couchbase.CouchbaseClient.Observe(String key, UInt64 cas, PersistTo persistTo, ReplicateTo replicateTo, ObserveKeyState persistedKeyState, ObserveKeyState replicatedState)
       at Couchbase.CouchbaseClient.ExecuteStore(StoreMode mode, String key, Object value, PersistTo persistTo, ReplicateTo replciateTo)
       at Couchbase.CouchbaseClient.ExecuteStore(StoreMode mode, String key, Object value, PersistTo persistTo)
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: