django-wkhtmltopdf: Error 'NoneType' object has no attribute 'endswith'

I’m trying to implement the basic example of this wrapper in my project using the url

url(r'^pdf/$', PDFTemplateView.as_view(template_name='my_template.html',
                                       filename='my_pdf.pdf'), name='pdf'),

my settings.py

WKHTMLTOPDF_CMD_OPTIONS = { ‘quiet’:True, } WKHTMLTOPDF_CMD = “/usr/local/bin/wkhtmltopdf/”

this is the error in the library