InputRequired(), message para value was not refered

[gainskills]

WTForms (2.1)
Flask-WTF (0.14.2)
Flask-Bootstrap (3.3.7.1)
Flask (0.12.2)

I define the login form by:

class LoginForm(FlaskForm):
    username = StringField('username', validators=[InputRequired(
        message="enter the username"), Length(min=6, max=16, message="Length should be more than 6 and less than 15")])
    <---ommited--->

route.py return the render html page, and the page was implemented by wtf.quick_form.

result:
Length messages field works, but InputRequired always shows the prompt: "Please fill out this field"

expectation:
parameter: the message of InputRequired should be loaded.