Unable to run mocha tests on firebase-functions, [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './lib/encoder' is not defined by "exports"

When I try and run a Mocha test script in a Firebase functions directory, I get this error:

Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './lib/encoder' is not defined by "exports" in /Users/[...redacted...]/functions/node_modules/firebase-functions/package.json

I am using Node 12 and my devDependencies are:

{
    "chai": "^4.2.0",
    "firebase-functions": "^3.18.1",
    "firebase-functions-test": "^0.2.3",
    "mocha": "^8.4.0"
}