Merge conflict error

While trying to merge conflicts given at I am getting the below error:

{"error":{"root_cause":[{"type":"action_request_validation_exception","reason":"Validation Failed: 1: mapping source is empty;"}],"type":"action_request_validation_exception","reason":"Validation Failed: 1: mapping source is empty;"},"status":400}
When I curl -XGET of the mapping source it does not return empty. Any pointers?

Can you provide the exact commands you are passing to ES?