Unsupported operation: Cannot add to a fixed-length list

Question

Hello I have a Flutter app and a UserModel which was generated by Amplify. In the UserModel I have two fields named followers and following which are lists. I wish to add the userID to them when the user presses the follow button. I defined them as

...
followers: [],
following: [],
...

By my knowledge these two lists should be growable but when I press the follow button to follow a specific user I get an error saying:

Unsupported operation: Cannot add to a fixed-length list

The error points me to the function where I described the logic of following a user which is

void followOneUser(
    User currentUser,
    User user,
    BuildContext context,
  ) async {
    if (currentUser.following.contains(user.id)) {
      currentUser.following.remove(user.id);
      user.followers.remove(currentUser.id);
    } else {
      currentUser.following.add(user.id);
                           ^^^^ // this is what causes the error
      user.followers.add(currentUser.id);
    }

    currentUser = currentUser.copyWith(following: currentUser.following);
    user = user.copyWith(followers: user.followers);

    final result = await ref.watch(userRepoProvider.notifier).followUser(
          currentUser,
          user,
        );

    result.fold(
      (l) => context.showError(
        l,
      ),
      (r) async {
        final res =
            await ref.watch(userRepoProvider.notifier).incrementFollowing(
                  currentUser,
                  user,
                );

        res.fold(
          (l) => context.showError(
            l,
          ),
          (r) => null,
        );
      },
    );
  }

I checked how the following and followers fields were described in UserModel and I couldn't figure out where they were defined as ungrowable Lists. Please help me fix this issue.

Thank you in advance