TypeError: Invalid magic value for BERT string

I'm trying to decode an erlang term but I'm getting this:

[3] pry(main)> BERT.decode('[{"x-max-hops",1},{"x-internal-purpose","federation"}]')
TypeError: Invalid magic value for BERT string
from /usr/local/Cellar/ruby/2.1.0/lib/ruby/gems/2.1.0/gems/bert-1.1.6/lib/bert/decoder.rb:8:in `decode'

Any ideas? - perhaps I'm not using the library right? - An example in the readme would be great, heh