How do i resolve this exception : User Username not found in membership store does not exist. - Blogs - Community Server 2007

Hi,

I am trying to get the forum threds from the community server .but i am facing the following exception

<Exception>
<InnerSource>CommunityServer.Components</InnerSource>
<InnerType>CommunityServer.Components.CSException</InnerType>
−
<InnerMessage>
CSException of type 'UserNotFound' was thrown: User Username not found in membership store does not exist.
</InnerMessage>
−
<InnerStack>
   at CommunityServer.Users.AddMembershipDataToUser(List`1 userList)
   at CommunityServer.Users.GetUsers(IList`1 userIds, Boolean isCacheable, Boolean fromCache)
   at CommunityServer.Users.AddUsersToCache(IList`1 userIds)
   at CommunityServer.Data.ForumsSqlDataProvider.GetThreads(ForumThreadQuery query)
   at CommunityServer.Discussions.Components.Threads.GetThreads(ForumThreadQuery query)
   at CommunityServer.WebServices.Services.Forums.ForumThreads.GetThreads(List`1 queryOptions)
   at CommunityServer.WebServices.Services.Forums.CSForumsService.ThreadGet(RequestInfo ri)
</InnerStack>
<Source>mscorlib</Source>
<Type>System.Reflection.TargetInvocationException</Type>
−
<Message>
Exception has been thrown by the target of an invocation.
</Message>
−
<Stack>
   at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
   at System.RuntimeMethodHandle.InvokeMethodFast(Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at CommunityServer.WebServices.Services.Common.Rest.RequestHandler.GetResponse(RequestInfo ri, MethodInfo method)
   at CommunityServer.WebServices.Services.Common.Rest.RequestHandler.ProcessRequest(HttpContext context)
</Stack>
</Exception>

Is this problem related to user membership?