rspec's should_receive with implicit subject

rspec string_spec.rb

» rspec string_spec.rb 
F

Failures:

  1) String 
     Failure/Error: it { should_receive(:size) }
       (#<RSpec::Core::ExampleGroup::Nested_1:0x000000013e67b8>).size(any args)
           expected: 1 time
           received: 0 times
     # ./string_spec.rb:4:in `block (2 levels) in <top (required)>'

Finished in 0.00091 seconds
1 example, 1 failure

Failed examples:

rspec ./string_spec.rb:4 # String 

string_spec.rb

describe String do
  subject { "asdf" }
  after { subject.size }
  it { should_receive(:size) }                                                                                                                                                                                                                                                  
end

Example showing unexpected spec failure as should_receive is not respecting implicit subject. This passes with explicit subject: subject.should_receive(:size)