Steps to reproduce:

1. Run Mongo 7+ and connect to it

docker run -d -p 27017:27017 --name mongo7-test mongo:7.0.2 --replSet mongo-test-rs 

2. Create an index and try dropping it via applyOps: dropIndexes, but omitting the ui field:

use test

rs.initiate()

db.testColl.createIndex({a: 1})

db.runCommand({applyOps: [{ op: 'c', ns: 'test.$cmd', o: { dropIndexes: 'testColl', index: 'a_1' }, o2: { v: 2, key: { a: 1 }, name: 'a_1' }}]}) 

Actual behavior:
Connection is closed (EOF).
On a server side, there is a fatal message in the log that a thread is terminated without an exception (backtrace is attached).

Expected behavior:

I'm not sure whether it should be able drop the index without specifying UUID (other commands work this way, e.g. see https://jira.mongodb.org/browse/SERVER-36951), but in Mongo 4.4-6.0 this runCommand has no effect: it completes successfully, but no indexes are dropped.

For Mongo 7+, this runCommand should not cause a crash as well.