Nested Stores and Nested Records cause error when committing changes

[blt04]

I have a simple data structure:

Product = SC.Record.extend {
  courses: SC.Record.toMany('Course', {nested: true})
}

I create a nested store:

chainStore = product.get('store').chain();
editProduct = chainStore.find(product);

When I make changes to editProduct and call commitChanges(), SC produces an error:

oldChildren is undefined

in registerChildToParet at store.js:1382

If I access one of the courses in the product record using objectAt before creating the nested store, everything works:

product.get('courses').objectAt(0)
chainStore = product.get('store').chain();
editProduct = chainStore.find(product);

What is the proper procedure for using nested stores on records with nested children?