InvalidCastException Though Same Base Class

The following code throws an InvalidCastException.

public static MachineProductCollection MachineProductsForMachine(
    MachineProductCollection MachineProductList, int MachineID)
{
    return (MachineProductCollection)
        MachineProductList.FindAll(c => c.MachineID == MachineID);
}

This surprises me since MachineProductCollection is merely a generic List of MachineProducts which is exactly FindAll() returns.  Here’s the full MachineProductCollection source code:

[Serializable]
public partial classMachineProductCollection :
        List<MachineProduct>
{
    public MachineProductCollection() { }
}

I resorted to the following which basically loops through the resulting List<MachineProduct> and adds them to my MachineProductCollection.  Obviously, I don’t like the required iteration. 

public static MachineProductCollection
    MachineProductForMachine(MachineProductCollection
    MachineProductList, int MachineID)
{
    MachineProductCollection result =
        new MachineProductCollection();

    foreach (MachineProduct machineProduct in
        MachineProductList.FindAll(c => c.MachineID == MachineID))
    {
        result.Add(machineProduct);
    }

    return result;
}

Documentation states an InvalidCastException is thrown when a failure occurs during an explicit reference conversion. Reference conversions are conversions from one reference type to another. While they may change the type of the reference, they never change the type or value of the conversion’s target. Casting objects from one type to another is a frequent cause for this exception.

Considering List<MachineProduct> is MachineProductCollection’s base, should this really be an InvalidCastException?