In the Jump Start tutorial "Web Forms Application - C#" step 14 "Running the Order Edit Form"

When saving the Order Edit form the Discount in the OrderDetail fails entityRule.

Stepping through the code in OrderDetailRules.cs:
discount = 0.0
mmType.IsEmpty(discount) = true

I do not understand why 0.0 fails. Should I not be able to have a discount of 0.0?

As you see in the tutorial documentation in step 14 the OrderDetail grid does show discount of 0 for each row.
And if the author tested this tutorial they would not have gotten the same error.

In my test the save fails due to the following rules message "Discount is required"

I know I could change the data, but I should be allowed to have a 0 discount.
Why is the Detail table being checked anyway? The data is readonly?

Any help is appreciated.