System.Web.Caching.ExpiresBucket.GetFreeExpiresEntry

Hi everyone. Just came across Cache error "Object reference not set to an instance of an object." and found that people talk about it but no one really knows why or how it happens and how can it be fix. thought stack might be helpful. 

at System.Web.Caching.ExpiresBucket.GetFreeExpiresEntry()

at System.Web.Caching.ExpiresBucket.AddCacheEntry(CacheEntry cacheEntry)

at System.Web.Caching.CacheExpires.Add(CacheEntry cacheEntry)

at System.Web.Caching.CacheSingle.UpdateCache(CacheKey cacheKey, CacheEntry newEntry, Boolean replace,CacheItemRemovedReason removedReason, Object& valueOld)

at System.Web.Caching.CacheMultiple.UpdateCache(CacheKey cacheKey, CacheEntry newEntry, Boolean replace, CacheItemRemovedReason removedReason, Object& valueOld)

at System.Web.Caching.CacheInternal.DoInsert(Boolean isPublic, String key, Object value, CacheDependency dependencies, DateTime utcAbsoluteExpiration, TimeSpan slidingExpiration, CacheItemPriority priority, CacheItemRemovedCallback onRemoveCallback, Boolean replace)

at System.Web.SessionState.InProcSessionStateStore.SetAndReleaseItemExclusive(HttpContext context, String id, SessionStateStoreData item, Object lockId, Boolean newItem)

at System.Web.SessionState.SessionStateModule.OnReleaseState(Object source, EventArgs eventArgs)

at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()

at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)