http output gives StoreContext#error_depth error

I have Logstash sending simple JSON messages to a test Bottle server I set up, and they work fine. However, when I try to set http output up with a more sophisticated URL as an endpoint, I get this error message 4 times in a row for every log entry that I send to Logstash:

_WARNING: unimplemented method called: StoreContext#error_depth_

The service whose REST API I'm using gets the JSON message correctly, but it would be nice to remove this error message. I'm using Logstash 1.5.0.

My http filter:

```
http {
headers => ["Authorization", "(the appropriate authorization)",
"Content-Type", "application/json"]
http_method => "post"
url => "(the appropriate URL)"
}
```