Iam getting data store exception was unhandled by user code

Hi all,
In this code am getting an error

private void CommitTransaction()
{
if (HttpContext.Current.Session == null)
{
return;
}

EntitiesModel dbContext =
HttpContext.Current.Session[CONTEXT_KEY] as EntitiesModel;
if (dbContext != null)
{
dbContext.SaveChanges();
}
}
in dbContext.SaveChanges() am getting an exceptiom

Am waiting for the reply