HTTP Runtime cache in ASP.Net web api

I am facing a problem in using HTTPRuntime Caching in ASP.net web api. i am adding a datatable in cache using below code.

function getDatatable(key) as datatable
 if HttpRuntime.Cache(Key) is nothing = True THEN
      // Get Datatable 
      HttpRuntime.Cache.Remove(Key)
      HttpRuntime.Cache.Add(Key, Obj, Nothing, System.DateTime.Now.AddMinutes(TimeInMinutes), System.Web.Caching.Cache.NoSlidingExpiration, Caching.CacheItemPriority.Normal, OnRemoveCallback)
      return datatable
 else
    return HttpRuntime.Cache(Key)    // this return as datatable
 End if
End function

now i am using this data table in another function

 Dim dtResult = getDatatable(key)
 if dtResult is nothing = false andAlso dtResult.rows.count > 0 then
    // Get Datatable columns
 End if

This is working perfectly fine with less number of users, but if there are concurrent request to this function then while getting column values it throw exception while getting value from datatable like converting dbnull to decimal. This happening for few requests and then works fine.

is there any way to track that what actually happens that in one request if application is getting value from datatable and at same time another request remove caching even then that datatable scope is limited in other function and must not effect.