Remote migration problem: uninitialized constant Enum

When I run 'rake remote:migrate' teh task fails with the error:

'uninitialized constant Enum'

Other rake remote tasks including rake deploy, update_code etc all work fine.

I get the same error whether I run 'rake remote:migrate' from my local
machine or when i run 'rake RAILS_ENV=production migrate' in the root
of my production app after ssh'ing in.

At the moment I'm using my MySQL client to keep the structure update
but would prefer to do this via migrations.

Any clues appreciated.