Caml Query to fetch the items where the Lookup Column value is not null

I have a list of employees in sharepoint. One of the columns in the Employee list is a lookup Column called Organization.
I am trying to write a CAML query to fetch all the employees where the Organization value is not empty. But, I receive all the items even if the value of Organization is empty.
This is what I am trying

           string querystring = string.empty;
           querystring = "<Where><IsNotNull><Field RefName='EmployeeOrganization_x0020_Organization' /></IsNotNull></Where>";

This query is returning all the items in the list.

SPQuery query = new SPQuery();
query.Query = "<Where><IsNotNull><FieldRef Name='EmployeeOrganization_x0020_Organization'/></IsNotNull></Where>";
DataTable dtemp = emplist.GetItems(query).GetDataTable();
  if (dtemp != null)
            {
               GridView1.DataSource = dtemp ;
                GridView1.DataBind();
            }