Interphinx undefined label warning

Following http://sphinx-doc.org/latest/ext/intersphinx.html I added sphinx.ext.intersphinx

to the extensions in mine conf.py

and then I defined the mapping against the Python 2.7 documentation:

intersphinx_mapping = {'python': ('http://docs.python.org/2.7', None)}

      

        
        
        
      

    

And I added this link to my reST source:

:ref:`comparison manual <python:comparisons>`

      

        
        
        
      

    

I also downloaded https://docs.python.org/2.7/objects.inv , which I decoded into reST to confirm that the comparison label exists in objects.inv:

:std:label:comparisons:
    :Link:  :std:label:`python:comparisons`
    :Domain:    Python
    :Version:   2.7
    :URL:   reference/expressions.html#comparisons
    :Title: Comparisons

      

        
        
        
      

    

However, when I try to build my documentation, I get the following warning:

WARNING: undefined label: python: comparisons (if the link does not have a signature must precede the section title)

and I am not getting any link in the html assembly file.

What am I doing wrong?