diff --git a/docs/extending.rst b/docs/extending.rst
index ed9de15..46572fa 100644
--- a/docs/extending.rst
+++ b/docs/extending.rst
@@ -33,6 +33,7 @@ serializes a boolean to the string ``true`` or ``false`` or the special
 .. code-block::  python
    :linenos:
 
+   from colander import Invalid
    from colander import null
 
    class Boolean(object):
@@ -40,12 +41,12 @@ serializes a boolean to the string ``true`` or ``false`` or the special
            if appstruct is null:
                return null
            if not isinstance(appstruct, bool):
-              raise Invalid(node, '%r is not a boolean')
+               raise Invalid(node, '%r is not a boolean')
            return appstruct and 'true' or 'false'
 
        def deserialize(self, node, cstruct):
            if cstruct is null:
-              return null
+               return null
            if not isinstance(cstruct, basestring):
                raise Invalid(node, '%r is not a string' % cstruct)
            value = cstruct.lower()