We are running an "applyOps" command with a single oplog entry using the Java driver -> an oplog entry that is an insertion into "system.indexes" collection. Before, when an index with a bad key (for example, a string) is in the oplog, it returns the following:

On mongoDB 3.5.10, it returns the following:

The fact that the error is "Unknown" renders us incapable of handling it gracefully.