Spec::Rails, "model.should have(1).error_on(:attribute)" passes when?

Spec::Rails people,
I'm curious about:
  - model.should have(1).error_on(:attribute)
displayed in the URL below:
  http://rspec.rubyforge.org/rspec-rails/1.2.2/classes/Spec/Rails/Exten
The Rspec peepcode screencast suggests that
model.should have(1).error_on(:attribute)
should pass if
model.send(:attribute)
returns nil
For me, I get an exception from the rspec script.
My work-around is simple.
Instead of:
  - model.should have(1).error_on(:attribute)
I Use:
  -model.send(:attribute).should be_nil
But now I have the question:
  - What kind of error causes:
    - model.should have(1).error_on(:attribute)
    - to pass ??
–b