Coding by intention, not accident

There is still something that doesn't sit well with me when I write kata. You're supposed to write the simplest code to make the test pass, but I feel sometimes this lead to coding by accident instead of intention.

For example, for the coin changer kata, when the user enters anything less than 0, I want to return an empty array indicating I have no change for the user.

So my test looks like this:

it "returns nothing when amount is less than the smallest coin value" do
  CoinChanger.get_change(0).should == []
end

For a simplest case, the code looks like this:

get_change(amount)
  []
end

If you read the code, do you think it expresses my intention well?

What happen when the amount is 1 cent? The program changes to this:

def get_change(amount)
  if (amount == 0)
    []
  else
    get_change(amount - 1) << 1
  end
end

But that's not what I mean. I really mean, if amount is 0, -1, or anything smaller than 0, I want it to return the empty array. So the code should really look like this:

def get_change(amount)
  if (amount <= 0)
    []
  else
    get_change(amount - 1) << 1
  end
end

When I write the simplest test case, sometimes, I forget the other condition. In this case, maybe I need to add another test case to test what happen when amount is less than 0. In general, I should be more careful and understand why my test is passing.

Note: This is not the best way to write coin changer. I just want to demonstrate what I was thinking.