Shouldn't test fixtures give an Error?

in the book we have this text fixtures for products controller:
@
one:
title: MyString
description: MyText
image_url: MyString
price: 9.99

two:
title: MyString
description: MyText
image_url: MyString
price: 9.99

ruby:
title: Programming Ruby 1.9
description: Ruby is the fastest growing and most exciting dynamic language out there. If you need to get working programs delivered fast, you should add Ruby to your toolbox.
price: 49.50
image_url: ruby.png
@

When we run the unit tests why don’t we have errors reporting that the image_url is not jpeg/png etc nor that we are trying to insert 2 products with the same name?