Error !

TypeError: cannot concatenate 'str' and 'NoneType' objects

<%inherit file="site" /> <%def name="main_content()"> <% book = None isbn = site.param('ISBN') if isbn: book = partials.getBook(isbn)
templates/fribokhandel/bookpage, line 5:
<%
templates/fribokhandel/site, line 100:
${self.main_content()}
/srv/dd-web02/limecms/fribokhandel2/fribokhandel201/local/lib/python2.7/site-packages/Mako-1.0.7-py2.7.egg/mako/runtime.py, line 899:
callable_(context, *args, **kwargs)
/srv/dd-web02/limecms/fribokhandel2/fribokhandel201/local/lib/python2.7/site-packages/Mako-1.0.7-py2.7.egg/mako/runtime.py, line 873:
_exec_template(inherit, lclcontext, args=args, kwargs=kwargs)
/srv/dd-web02/limecms/fribokhandel2/fribokhandel201/local/lib/python2.7/site-packages/Mako-1.0.7-py2.7.egg/mako/runtime.py, line 838:
**_kwargs_for_callable(callable_, data))
/srv/dd-web02/limecms/fribokhandel2/fribokhandel201/local/lib/python2.7/site-packages/Mako-1.0.7-py2.7.egg/mako/template.py, line 462:
return runtime._render(self, self.callable_, args, data)
/var/limecms/fribokhandel2/fribokhandel201/fribokhandel2-inst/Products/LimeTemplate/LimeTemplate.py, line 704:
html = mytemplate.render(object=object, site=site, request=self.REQUEST, extra=extra.get('kwargs', None), **extra)