S3 storage fails with fog 0.7.2 + carrierwave master branch

Question

When trying to store a file I get the following:

TypeError: can't convert nil into String
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/fog-0.7.2/lib/fog/core/hmac.rb:23:in `digest'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/fog-0.7.2/lib/fog/core/hmac.rb:23:in `block in setup_sha1'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/fog-0.7.2/lib/fog/core/hmac.rb:15:in `call'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/fog-0.7.2/lib/fog/core/hmac.rb:15:in `sign'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/fog-0.7.2/lib/fog/storage/aws.rb:309:in `signature'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/fog-0.7.2/lib/fog/storage/aws.rb:317:in `request'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/fog-0.7.2/lib/fog/storage/requests/aws/put_object.rb:43:in `put_object'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/fog-0.7.2/lib/fog/storage/models/aws/file.rb:119:in `save'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/fog-0.7.2/lib/fog/core/collection.rb:50:in `create'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/bundler/gems/carrierwave-d2cb99103a4f/lib/carrierwave/storage/fog.rb:229:in `store'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/bundler/gems/carrierwave-d2cb99103a4f/lib/carrierwave/storage/fog.rb:79:in `store!'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/bundler/gems/carrierwave-d2cb99103a4f/lib/carrierwave/uploader/store.rb:59:in `block in store!'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/bundler/gems/carrierwave-d2cb99103a4f/lib/carrierwave/uploader/callbacks.rb:17:in `with_callbacks'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/bundler/gems/carrierwave-d2cb99103a4f/lib/carrierwave/uploader/store.rb:58:in `store!'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/bundler/gems/carrierwave-d2cb99103a4f/lib/carrierwave/mount.rb:324:in `store!'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/bundler/gems/carrierwave-d2cb99103a4f/lib/carrierwave/mount.rb:214:in `store_poster!'
... 3 levels...
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/activerecord-3.0.7/lib/active_record/validations.rb:43:in `save'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/activerecord-3.0.7/lib/active_record/attribute_methods/dirty.rb:21:in `save'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/activerecord-3.0.7/lib/active_record/transactions.rb:240:in `block (2 levels) in save'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/activerecord-3.0.7/lib/active_record/transactions.rb:292:in `block in with_transaction_returning_status'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/activerecord-3.0.7/lib/active_record/connection_adapters/abstract/database_statements.rb:139:in `transaction'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/activerecord-3.0.7/lib/active_record/transactions.rb:207:in `transaction'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/activerecord-3.0.7/lib/active_record/transactions.rb:290:in `with_transaction_returning_status'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/activerecord-3.0.7/lib/active_record/transactions.rb:240:in `block in save'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/activerecord-3.0.7/lib/active_record/transactions.rb:251:in `rollback_active_record_state!'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/activerecord-3.0.7/lib/active_record/transactions.rb:239:in `save'
    from (irb):2
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.7/lib/rails/commands/console.rb:44:in `start'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.7/lib/rails/commands/console.rb:8:in `start'
    from /Users/nick/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.7/lib/rails/commands.rb:23:in `<top (required)>'
    from script/rails:6:in `require'

I've tried to debug manually and I wonder whether the issue ties in with the fact @content_type is being set as nil, although I haven't been able to deduce much further than that.

This is an s3 (or fog, I don't have any other providers) specific issue and my project works fine when using :file storage. Thoughts?