Fatal error: call to member function on a non-object

Hi. I've just installed this module and it looks very promising. Unfortunately, when requesting a node 'Edit' page, I get a white screen with this error:

Fatal error: Call to a member function condition() on a non-object in /path/to/site/modules/simple_access/simple_access.module on line 637

I also get this error when viewing the content type 'Edit' page but, this time, with the form correctly displayed.

Notice: Undefined index: sa in simple_access_form_node_type_form_alter() (line 385 of /path/to/site/modules/simple_access/simple_access.module).

I'm not sure how do diagnose it further.