Firestoreのドキュメント名が数字のとき、「Value for argument “documentPath” is not a valid resource path.」と怒られた時のエラー対処法

const subject_id = 3681;
firestoreDb.collection('subject_data').doc(subject_id)
                     .get().then(doc => {})

(node:79393) UnhandledPromiseRejectionWarning: Error: Value for argument "documentPath" is not a valid resource path. Path must be a non-empty string.
    at Object.validateResourcePath (/Users/...../node_modules/@google-cloud/firestore/build/src/path.js:406:15)
    at CollectionReference.doc (/Users/...../node_modules/@google-cloud/firestore/build/src/reference.js:1982:20)
    at updateDataFromFirebase (/Users/...../test_updateDataFromFirebase.js:21:63)
    at Object.<anonymous> (/Users...../test_updateDataFromFirebase.js:16:1)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
(node:79393) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:79393) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

const subject_id = new Number(3681).toString();
firestoreDb.collection('subject_data').doc(subject_id)
                     .get().then(doc => {})