Index: wp-includes/capabilities.php
===================================================================
--- wp-includes/capabilities.php	(revision 18407)
+++ wp-includes/capabilities.php	(working copy)
@@ -877,6 +877,12 @@
 
 		$post_type = get_post_type_object( $post->post_type );
 
+		if ( ! $post_type) {
+			$cap = 'edit_published_posts';
+			$caps[] = 'edit_published_posts';
+			break;
+		}
+
 		if ( ! $post_type->map_meta_cap ) {
 			$caps[] = $post_type->cap->$cap;
 			// Prior to 3.1 we would re-call map_meta_cap here.