Rails - Apr 2009 - NoMethodError in StoreController#add_to_cart

Working through the Agile Web Development w/ Rails book on p114.  I can
add single items of each to the cart but when I try to add a second item
I get the following error:

 NoMethodError in StoreController#add_to_cart

You have a nil object when you didn''t expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.+

Any idea why this is happening?  Thanks.

Frank.
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