Cannot overlay an object of type "System.Double" on type "System.Nullable`1 [System.Single]"

Why am I getting the above error in this LINQ query - and how can I solve it?

System.Nullable<float> totalFreight =
    (from cust in db.Customers
       join ord in db.Orders on cust.ordID equals ord.OrdID
      select ord.Freight).Sum();

      

        
        
        
      

    

Note : the Freight

attribute / column in the entity / table Orders is of type float?

. I am using EF Core 1.1

with VS2015

.

UPDATE . I am trying to follow this and this MSDN Examples.

UPDATE 2 . I have highlighted the problem as follows. Moreover, I checked by hovering over ord.Freight

that Freight is indeed a float:

var totalFreight =
        from cust in db.Customers
           join ord in db.Orders on cust.ordID equals ord.OrdID
          select ord.Freight

var testSum = totalFreight.Sum();

      

        
        
        
      

    

But you still get the same error, this time on the line: var testSum = totalFreight.Sum();