Identical hashes returned from lambda behaving differently - ruby

Question

I'm using Paperclip for uploads to a Rails 4 image server. We want to track different image sizes and be able to generate new sizes on the fly if an existing style with the requested size does not exist.
To do this we have, in addition to a Photo model, a Size model belonging to Photo. I'm using a lambda to generate a hash object from photo.instance.sizes and pass that into Paperclip's :styles parameter. The hash gets generated just fine and when I log it to standard output I see a hash with the default size in Paperclip's desired format. {:medium=>"300x300#"} (worth noting the keys in the generated hash are symbolic, Paperclip doesn't like string keys)
When this hash is being returned from the lambda, no additional styles are generated and only the original resolution is saved. However, when I return a hard-coded hash (default var in code below) with the same contents, the additional style "medium" is generated (along with the original).
I made sure the hard coded hash was exactly the same as the one generated from photo.instance.sizes by logging an === comparison which returns true.
Here's the has_attached_file from the Photo model:
has_attached_file :asset,
:styles => lambda { |photo|
# Empty hash will contain paperclip formatted style options
styles = {}
# Get instance of model and iterate over its sizes
photo.instance.sizes.each do |size|
# Paperclip style format { :style_name => "WIDTHxHEIGHT#" } trailing pound means crop and retain aspect ratio
styles[size.name.to_sym] = "#{size.width}x#{size.height}#"
end
# default = { medium: "300x300#" }
styles
},
:storage => :s3,
:path => "/assets/:idhash/:style.:extension",
:s3_credentials => {:bucket => ENV["AWS_BUCKET"], :access_key_id => ENV["AWS_ACCESS_KEY_ID"], :secret_access_key => ENV["AWS_ACCESS_KEY_SECRET"]}
Any insight (or an alternative) would be greatly appreciated!