```ruby
require File.expand_path('../../../spec_helper', __FILE__)
require File.expand_path('../fixtures/classes', __FILE__)

ruby_version_is "1.9" do
  describe "Math.cbrt" do
    it "returns a float" do
      Math.cbrt(1).should be_kind_of(Float)
    end

    it "returns the cubic root of the argument" do
      Math.cbrt(1).should == 1.0
      Math.cbrt(8.0).should == 2.0
      Math.cbrt(-8.0).should == -2.0
      Math.cbrt(3).should be_close(1.44224957030741, TOLERANCE)
    end

    it "raises a TypeError if the argument cannot be coerced with Float()" do
      lambda { Math.cbrt("foobar") }.should raise_error(TypeError)
    end

    it "raises a TypeError if the argument is nil" do
      lambda { Math.cbrt(nil) }.should raise_error(TypeError)
    end

    it "accepts any argument that can be coerced with Float()" do
      Math.cbrt(MathSpecs::Float.new).should be_close(1.0, TOLERANCE)
    end
  end
end
```

Change to `be_an_instance_of` as per @brixen's feedback.
user
```ruby
require File.expand_path('../../../spec_helper', __FILE__)
require File.expand_path('../fixtures/classes', __FILE__)

ruby_version_is "1.9" do
  describe "Math.cbrt" do
    it "returns a float" do
      Math.cbrt(1).should be_an_instance_of(Float)
    end

    it "returns the cubic root of the argument" do
      Math.cbrt(1).should == 1.0
      Math.cbrt(8.0).should == 2.0
      Math.cbrt(-8.0).should == -2.0
      Math.cbrt(3).should be_close(1.44224957030741, TOLERANCE)
    end

    it "raises a TypeError if the argument cannot be coerced with Float()" do
      lambda { Math.cbrt("foobar") }.should raise_error(TypeError)
    end

    it "raises a TypeError if the argument is nil" do
      lambda { Math.cbrt(nil) }.should raise_error(TypeError)
    end

    it "accepts any argument that can be coerced with Float()" do
      Math.cbrt(MathSpecs::Float.new).should be_close(1.0, TOLERANCE)
    end
  end
end
```
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