Error occurs when using s3 as sole input

I had a working configuration using file and s3 inputs.

When I removed the file input, I began receiving this message when starting logstash :

Using milestone 1 input plugin 's3'. This plugin should work, but would benefit from use by folks like you. Please let us know if you find bugs or have suggestions on how to improve this plugin. For more information on plugin milestones, see http://logstash.net/docs/1.2.1/plugin-milestones {:level=>:warn}
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The error reported is:
uninitialized constant Digest::MD5

The error did not occur when I restored my file input back to the configuration.

I stripped down the configs to the minimum set that trigger the same error condition :

The s3 bucket and credentials provided are invalid, but they produce the same MD5 error that I produced with valid credentials / bucket.

Adding a file input to the configuration before the s3 definition in the above config results an AWS authentication error, as expected. Adding it after the s3 definition results in the original MD5 error.

I have a workaround to this bug - keeping a no-op file input in my configuration.