400 error from _reindex

I am attempting to use reindex to effect the conversion of a single field from a string to a float. I have run this on several indices without issue, but in my most recent attempt, I get the following error after 29,000 documents have been processed into the new index:

{
  "error" : {
    "root_cause" : [
      {
        "type" : "parse_exception",
        "reason" : "request body is required"
      }
    ],
    "type" : "parse_exception",
    "reason" : "request body is required"
  },
  "status" : 400
}

The mapping is a simple source/dest mapping:

POST _reindex
{
  "source": {
    "index": "staging-6.7.1-2021.03.02"
  },
  "dest": {
    "index": "staging-6.7.1-2021.03.02-reindex-time-elapsed"
  }
}

I can post the definition of the new index, but it is very long. Basically, I took the output from a GET on the source index, removed from the settings section anything that wasn't appropriate (provided_name, creation_date, etc.) and then changed the data type of the field I cared about. Like I said, this has worked for me.

How can I dig into the source of the 400 error that I am receiving?

Thank you,

Don