TypeError: expected string or buffer

I got following error while clicking on a product from front end

Uncaught Server Exception
Traceback (most recent call last):
File “/srv/bench/erpnext/apps/frappe/frappe/website/render.py”, line 46, in render
data = render_page_by_language(path)
File “/srv/bench/erpnext/apps/frappe/frappe/website/render.py”, line 143, in render_page_by_language
return render_page(path)
File “/srv/bench/erpnext/apps/frappe/frappe/website/render.py”, line 159, in render_page
return build(path)
File “/srv/bench/erpnext/apps/frappe/frappe/website/render.py”, line 166, in build
return build_page(path)
File “/srv/bench/erpnext/apps/frappe/frappe/website/render.py”, line 188, in build_page
html = frappe.get_template(context.template).render(context)
File “/srv/bench/erpnext/env/local/lib/python2.7/site-packages/jinja2/environment.py”, line 1008, in render
return self.environment.handle_exception(exc_info, True)
File “/srv/bench/erpnext/env/local/lib/python2.7/site-packages/jinja2/environment.py”, line 780, in handle_exception
reraise(exc_type, exc_value, tb)
File “/srv/bench/erpnext/apps/erpnext/erpnext/./templates/generators/item/item.html”, line 1, in top-level template code
{% extends “templates/web.html” %}
File “/srv/bench/erpnext/apps/frappe/frappe/./templates/web.html”, line 1, in top-level template code
{% extends base_template_path %}
File “/srv/bench/erpnext/apps/frappe/frappe/./templates/base.html”, line 68, in top-level template code
{% block content %}
File “/srv/bench/erpnext/apps/frappe/frappe/./templates/web.html”, line 52, in block “content”
{{ main_content() }}
File “/srv/bench/erpnext/env/local/lib/python2.7/site-packages/jinja2/runtime.py”, line 579, in _invoke
rv = self._func(*arguments)
File “/srv/bench/erpnext/apps/frappe/frappe/./templates/web.html”, line 28, in template
{%- block page_content -%}{%- endblock -%}
File “/srv/bench/erpnext/apps/erpnext/erpnext/./templates/generators/item/item.html”, line 15, in block “page_content”
{% include “templates/generators/item/item_details.html” %}
File “/srv/bench/erpnext/apps/erpnext/erpnext/./templates/generators/item/item_details.html”, line 12, in top-level template code
{% if frappe.utils.strip_html(doc.web_long_description) %}
File “/srv/bench/erpnext/apps/frappe/frappe/utils/data.py”, line 603, in strip_html
return _striptags_re.sub("", text)
TypeError: expected string or buffer