Bulk api mapper exception


We have encountered one problem and I am not sure why exactly.
We are using bulk api to index data. Suddenly we see the below error.

Failed to commit transaction. Transaction rolled back.
stack_trace: org.apache.flume.EventDeliveryException: failure in bulk execution:
message [MapperParsingException[failed to parse]; nested: IllegalArgumentException[mapper [param_value] of different type, current_type [string], merged_type [date]];]

Though, if I check the current mapping of the index, I do not see the field "param_value" as no value is yet put into the index and possibly the bulk request which failed was trying for the first time to insert but failed.

As the mapping doesn't exist, why do I get a conflict?

I am only guessing here, but can it be that the bulk request has data in the param_value field, one of which can be mapped as a string and other as date? And for the field being written to for the first time for the index, there is mapping conflict within bulk request?

We have 8 nodes where this problem occurred.
I tried to reproduce this on single node setup but i couldn't.

Can you please let me know how the bulk api and mapping happens?
What may be the possible cause for the problem and how can we get around it?

What version are you on?
How does flume fit into things?

Apologies, I had not mentioned the version. ES 2.3.1

We are using apache-flume to put data into ES, with a modified sink plugin to work with ES 2.3.1