Shoulda rspec matchers :on => :create

Question

I am using some of the Shoulda rspec matchers to the test my model, one of them being:

describe Issue do
  it { should_not allow_value("test").for(:priority) }
end

My problem with this is that my validation in my model looks like this:

validates_format_of :priority, :with => /^(Low|Normal|High|Urgent)$/, :on => :update

So when running this test I get:

1) 'Issue should not allow priority to be set to "test"' FAILED
   Expected errors when priority is set to "test", got errors: category is invalid (nil)title can't be blank (nil)profile_id can't be blank (nil)

The validation isn't being triggered because it only runs on an update, how can I use these shoulda matchers on an update vs. a create?