Undefined method for NilClass in buffer_flush

Just encountered this error in my logs, Logstash crashed immediately afterwards. Not sure if it's an output-sqs or a stud / buffer issue.

May 11 15:22:09 logstash.err: NoMethodError: undefined method `[]' for nil:NilClass
May 11 15:22:09 logstash.err: buffer_flush at /opt/logstash/vendor/bundle/jruby/1.9/gems/stud-0.0.22/lib/stud/buffer.rb:186
May 11 15:22:09 logstash.err: close at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-sqs-2.0.2/lib/logstash/outputs/sqs.rb:135
May 11 15:22:09 logstash.err: do_close at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-2.1.3-java/lib/logstash/plugin.rb:36
May 11 15:22:09 logstash.err: each at org/jruby/RubyArray.java:1613
May 11 15:22:09 logstash.err: outputworker at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-2.1.3-java/lib/logstash/pipeline.rb:282
May 11 15:22:09 logstash.err: each at org/jruby/RubyArray.java:1613
May 11 15:22:09 logstash.err: outputworker at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-2.1.3-java/lib/logstash/pipeline.rb:281
May 11 15:22:09 logstash.err: start_outputs at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-2.1.3-java/lib/logstash/pipeline.rb:194