Firestore security rules - Property resource is undefined on object - firebase

Question

For some reason when I try to use the resource.data I get an error saying that resource is undefined. In this case it occurs in the notUpdating() function. Why is this and how can I fix it?
I'm new to firebase - Thanks for the help!
Error: simulator.rules line [12], column [28]. Property resource is undefined on object.
Rules:
rules_version = '2';
service cloud.firestore {
match /databases/{database}/documents {
match /users/{userId} {
function authed() {
return request.auth != null
}
function matchesUser() {
return request.auth.uid == userId
}
function notUpdating(field) {
return !(field in request.resource.data) || resource.data[field] == request.resource.data[field]
}
match /folders/{docId} {
allow read: if authed() && matchesUser()
allow create: if authed() && matchesUser()
}
match /files/{docId} {
allow read: if authed() && matchesUser()
allow create: if authed() && matchesUser()
allow update: if authed() && matchesUser() && notUpdating("userId")
}
}
}
}
My code for adding and updating a file:
function addFile() {
if (!filePathInp.value) return;
const fileName = filePathInp.value.split("\\").pop();
const fileRef = ref(storage, `users/${$user.uid}/files/${fileName}`);
const uploadTask = uploadBytesResumable(fileRef, filePathInp.value);
uploadTask.on(
"state_changed",
(snapshot) => {},
(error) => {
console.log(error);
},
() => {
// upload complete
getDownloadURL(uploadTask.snapshot.ref).then((downloadURL) => {
const files = collection(firestore, `users/${$user.uid}/files`);
const filesQuery = query(
files,
where("name", "==", fileName),
where("userId", "==", $user.uid),
where("parentPath", "==", path)
);
getDocs(filesQuery).then((doc) => {
if (doc.empty) {
addDoc(files, {
url: downloadURL,
name: fileName,
createdAt: new Date(),
parentPath: path,
userId: $user.uid,
});
} else {
updateDoc(doc.docs[0].ref, {
url: downloadURL,
});
}
});
});
}
);
filePathInp.value = "";
}