Upgrading to Elastic 1.5.1 from 1.4.2 started raising ClassCastException

Hi there!

I have a small cluster that was running fine on Elastic 1.4.2 and due to
the stability improvements mentioned on 1.5 we decided to upgrade. The
upgrade was done as a rolling upgrade (one instance at a time) and
everything appeared to be going fine.

We don't use Logstash, but we do split our indexes by day. Once the day
changed and a new index was automatically created we started getting a lot
of these messages:

[2015-05-11 13:35:49,852][DEBUG][action.search.type ] [elastic01] [
events_2015-03-17][2], node[8GzJ8I3YSAChFxmK9a4drQ], [R], s[STARTED]: Failed
to execute [org.elasticsearch.action.search.SearchRequest@51be6ea0]
lastShard [true]
org.elasticsearch.transport.RemoteTransportException: [elastic03][inet[/
192.168.171.33:9300]][indices:data/read/search[phase/query]]
Caused by: java.lang.ClassCastException
[2015-05-11 13:35:49,853][DEBUG][action.search.type ] [elastic01] [
events_2015-03-17][0], node[8GzJ8I3YSAChFxmK9a4drQ], [P], s[STARTED]: Failed
to execute [org.elasticsearch.action.search.SearchRequest@51be6ea0]
lastShard [true]
org.elasticsearch.transport.RemoteTransportException: [elastic03][inet[/
192.168.171.33:9300]][indices:data/read/search[phase/query]]
Caused by: java.lang.ClassCastException

We didn't modify our code at all, just upgraded the version of
Elasticsearch running on the cluster.

Is this message harmless? What can I do to fix it or help you guys debug
this?

Thanks in advance,

Eduardo Castegnaro.

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There is usually a stack trace after the line "Caused by: java.lang.ClassCastException", can you include the full stack?