This seems to be an encoding issue, when the driver attempts to serialize to BSON it gets an encoding mismatch. Here's a chunk of stack trace:

Encoding::CompatibilityError: incompatible character encodings: ASCII-8BIT and UTF-8
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/protocol/serializers.rb:77:in `serialize'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/protocol/message.rb:155:in `block in serialize_fields'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/protocol/message.rb:141:in `each'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/protocol/message.rb:141:in `serialize_fields'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/protocol/message.rb:70:in `serialize'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/server/connection.rb:123:in `block in write'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/server/connection.rb:122:in `each'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/server/connection.rb:122:in `write'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/server/connectable.rb:66:in `block in dispatch'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/loggable.rb:44:in `log'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/loggable.rb:65:in `log_debug'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/server/connectable.rb:65:in `dispatch'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/operation/read/query.rb:61:in `block in execute_message'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/server/connection_pool.rb:99:in `with_connection'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/server/context.rb:63:in `with_connection'
from /app/vendor/bundle/ruby/2.1.0/gems/mongo-2.0.4/lib/mongo/operation/read/query.rb:60:in `execute_message'