Flask-WTF在应该失败时通过验证

from flask_wtf import FlaskForm
from wtforms import StringField, SubmitField
from wtforms.validators import DataRequired, Length

class MyForm(FlaskForm):
    username = StringField('Username', validators=[DataRequired(), Length(min=4, max=20)])
    submit = SubmitField('Submit')

@app.route('/form', methods=['GET', 'POST'])
def form():
    form = MyForm()
    if form.validate_on_submit():
        # 表单数据验证成功，执行相关操作
        return 'Success!'
    return render_template('form.html', form=form)

<form method="POST" action="/form">
    {{ form.csrf_token }}
    {{ form.username.label }} {{ form.username() }}
    {% for error in form.username.errors %}
        <span class="error">{{ error }}</span>
    {% endfor %}
    {{ form.submit() }}
</form>