AttributeError: 'bool' object has no attribute '__call__'

I am working on the templates in flask application. While trying to work on the authentication for when a user is logged in or not, I keep recieving the error

AttributeError: 'bool' object has no attribute '__call__'

I have this written in my templates/base.html

<li><a href="">Sign In</a></li>

I have no idea while this is happening, I have also written this in the view function for calling the template, if this might be the reason,

from flask import current_user

but still it does the same thing.

What should I do?