Flask wtforms - 'UnboundField' object is not callable, dynamic field won't init properly

app.py

from flask import Flask, render_template
from flask_wtf import FlaskForm
from wtforms import StringField, SubmitField, FieldList, FormField

app = Flask(__name__)
app.config['SECRET_KEY'] = 'apple pie'


class BookForm(FlaskForm):
    book = StringField('book title')


class LibraryForm(FlaskForm):
    def __init__(self, min_entries=0, *args, **kwargs):
        super(LibraryForm, self).__init__(*args, **kwargs)
        self.books = FieldList(FormField(BookForm), min_entries=min_entries)

    library = StringField('Library name')
    books = FieldList(FormField(BookForm), min_entries=3)
    submit = SubmitField('Submit')


@app.route('/book', methods=['GET', 'POST'])
def book():
    form = LibraryForm(min_entries=5)
    if form.validate_on_submit():
        return 'aww yeah'
    return render_template('books.html', form=form)

books.html

<html>
<form method="POST" action="">
    {{ form.hidden_tag() }}
    <div>{{ form.library.label }}: {{ form.library() }}</div>
    <div>{{ form.books.label }}: {{ form.books() }}</div>
    <div>{{ form.submit.label }}: {{ form.submit() }}</div>
</html>

My goal is to make my form init with a flexible number of entries, such as in the example shown in the link. Whenever I run my code I get the following error:

TypeError: 'UnboundField' object is not callable

When I comment out the init function, the form works as expected, apart from the non-flexible amount of Book Fields. I've spent quite some time looking for answers, but couldn't find any which solved the problem.

example of how it should look

Any help is appeciated!

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