Summary

Top-level dollar signs are not supported in mongodb versions < 4.2, per the legacy documentation. A recent change to key management API (in DRIVERS-2017) added a dependency on aggregation pipeline style update operators which are not supported until 4.2. Apps that attempt to use removeKeyAltName() on MongoDB <4.2 will receive a cryptic error like this:

The dollar ($) prefixed field '$set' in '0.$set' is not valid for storage.

This particular error will occur when running something like the following for a mongodb version < 4.2:

def remove_alt_name(name):
    return coll.find_one_and_update({"keyAltNames": name}, [{"$set": { "x": 1 }}])

One potential update could ensure the resulting error reads

FLE requires mongodb version 4.2+: The dollar ($) prefixed field '$set' in '0.$set' is not valid for storage.

Motivation

Who is the affected end user?

Anyone using version < 4.2 for aggregating data.

How does this affect the end user?

Cosmetic, clearer error messages for resolution.