diff --git a/docs/topics/spiders.rst b/docs/topics/spiders.rst
index 0e473709a..af676fccd 100644
--- a/docs/topics/spiders.rst
+++ b/docs/topics/spiders.rst
@@ -72,6 +72,8 @@ scrapy.Spider
        spider that crawls ``mywebsite.com`` would often be called
        ``mywebsite``.
 
+       .. note:: In Python 2 this must be ASCII only.
+
    .. attribute:: allowed_domains
 
        An optional list of strings containing domains that this spider is