Timestamp parsing fails with "cannot convert instance of class org.jruby.RubyObject to class java.lang.String"

Hi, using the logstash package on debian wheezy with the following versions:
logstash 1.5.4-1
openjdk-7-jre-headless:amd64 7u79-2.5.6-1~deb7u1

I'm getting the following message:

```
{:timestamp=>"2015-08-25T14:25:17.945000+1000", :message=>"Failed parsing date from field", :field=>"timestamp", :value=>"2015-08-25T04:25:17.000Z", :exception=>"cannot convert instance of class org.jruby.RubyObject to class java.lang.String", :config_parsers=>"yyyy-MM-dd HH:mm:ss.SSS,yyyy-MM-dd HH:mm:ss,SSS,yyyy-MM-dd HH:mm:ss,yyyy/MM/dd HH:mm:ss,MMM d HH:mm:ss,MMM dd HH:mm:ss,dd/MMM/yyyy:HH:mm:ss Z,yyyy-MM-dd HH:mm:ss.SSSZ,yyyy-MM-dd'T'HH:mm:ss.SSSZ,yyyy-MM-dd'T'HH:mm:ssZ,E MMM dd HH:mm:ss yyyy Z", :config_locale=>"default=en_AU", :level=>:warn}
```

At this point I'm not quite sure what's causing it.
I'm suspecting the following tidbit in the config but cannot be certain (namely the _target_ part):

```
date {
match => ["timestamp",
"yyyy-MM-dd HH:mm:ss.SSS",
"yyyy-MM-dd HH:mm:ss,SSS",
"yyyy-MM-dd HH:mm:ss",
"yyyy/MM/dd HH:mm:ss",
"MMM d HH:mm:ss",
"MMM dd HH:mm:ss",
"dd/MMM/yyyy:HH:mm:ss Z",
"yyyy-MM-dd HH:mm:ss.SSSZ",
"yyyy-MM-dd'T'HH:mm:ss.SSSZ",
"yyyy-MM-dd'T'HH:mm:ssZ",
"E MMM dd HH:mm:ss yyyy Z"
]
target => "timestamp"
timezone => 'Australia/Melbourne'
}
```

Any ideas?