xhtml2pdf pdf rendering errors

>  File "/project/.venv/lib/python2.7/site-packages/easy_pdf/rendering.py", line 139, in render_to_pdf
>    return html_to_pdf(content, encoding, **kwargs)
>  File "/project/.venv/lib/python2.7/site-packages/easy_pdf/rendering.py", line 65, in html_to_pdf
>    link_callback=link_callback, **kwargs)
>  File "/project/.venv/lib/python2.7/site-packages/xhtml2pdf/document.py", line 89, in pisaDocument
>    encoding, context=context, xml_output=xml_output)
>  File "/project/.venv/lib/python2.7/site-packages/xhtml2pdf/document.py", line 57, in pisaStory
>    pisaParser(src, context, default_css, xhtml, encoding, xml_output)
>  File "/project/.venv/lib/python2.7/site-packages/xhtml2pdf/parser.py", line 685, in pisaParser
>    context.parseCSS()
>  File "/project/.venv/lib/python2.7/site-packages/xhtml2pdf/context.py", line 498, in parseCSS
>    self.css = self.cssParser.parse(self.cssText)
>  File "/project/.venv/lib/python2.7/site-packages/xhtml2pdf/w3c/cssParser.py", line 434, in parse
>    src, stylesheet = self._parseStylesheet(src)
>  File "/project/.venv/lib/python2.7/site-packages/xhtml2pdf/w3c/cssParser.py", line 522, in _parseStylesheet
>    src, stylesheetImports = self._parseAtImports(src)
>  File "/project/.venv/lib/python2.7/site-packages/xhtml2pdf/w3c/cssParser.py", line 606, in _parseAtImports
>    stylesheet = self.cssBuilder.atImport(import_, mediums, self)
>  File "/project/.venv/lib/python2.7/site-packages/xhtml2pdf/w3c/css.py", line 874, in atImport
>    return cssParser.parseExternal(import_)
>  File "/project/.venv/lib/python2.7/site-packages/xhtml2pdf/context.py", line 372, in parseExternal
>    cssFile = self.c.getFile(cssResourceName, relative=self.rootPath)
>  File "/project/.venv/lib/python2.7/site-packages/xhtml2pdf/context.py", line 822, in getFile
>    return getFile(self._getFileDeprecated(name, relative))
>  File "/project/.venv/lib/python2.7/site-packages/xhtml2pdf/util.py", line 635, in getFile
>    file = pisaFileObject(*a, **kw)
>  File "/project/.venv/lib/python2.7/site-packages/xhtml2pdf/util.py", line 516, in __init__
>    uri = uri.encode('utf-8')
>AttributeError: 'NoneType' object has no attribute 'encode'