How to specify a specific version for a pod in combination with modular_header?

I have this in a pod file

    pod 'Realm', :modular_headers => true

And want to specify a particular version to use. Ordinarily without the modular_headers section I would do this:

    pod 'Realm', 'N.N.N'

But I can't get that to work in combination with the modular_headers bit i.e. all of these fail:

    pod 'Realm', 'N.N.N', :modular_headers => true    pod 'Realm', 'N.N.N' :modular_headers => true     pod 'Realm', :modular_headers => true, N.N.N'    pod 'Realm', :modular_headers => true 'N.N.N'

All the example above