Rails - Apr 2006 - weird error when running my unit tests

How can this be possible:

I have two tests:

  def find_all_active_and_inactive
    assert_equal Concept.find_all_active_and_inactive.size, 
@concepts.size
  end

and

  def test_find_all_inactive
    assert_equal Concept.find_all_inactive.size, (@concepts.size - 
Concept.find_all.size)
  end

Concept.find_all returns 2 concepts (I overwrote the method to return 
only the active concepts)
Concept.find_all_inactive returns 1 concept
Concept.find_all_active_and_inactive returns 3 concepts

And @concepts.size returns 3

When I run the first test, there is no error.

When I run the second test, I get this error:

1) Error:
test_find_all_inactive(ConceptTest):
NoMethodError: You have a nil object when you didn''t expect it!
You might have expected an instance of Array.
The error occured while evaluating nil.size
    concept_test.rb:36:in `test_find_all_inactive''

I dont understand! Why is @concepts.size working in the first test and 
not in the second?

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