Index: wp-includes/capabilities.php
===================================================================
--- wp-includes/capabilities.php	(revision 24220)
+++ wp-includes/capabilities.php	(working copy)
@@ -1022,6 +1022,11 @@
 		}
 
 		$post_type = get_post_type_object( $post->post_type );
+		
+		if ( is_null( $post_type ) ) {
+			$caps[] = 'do_not_allow';
+			break;
+		}
 
 		if ( ! $post_type->map_meta_cap ) {
 			$caps[] = $post_type->cap->$cap;
@@ -1072,6 +1077,11 @@
 		}
 
 		$post_type = get_post_type_object( $post->post_type );
+		
+		if ( is_null( $post_type ) ) {
+			$caps[] = 'do_not_allow';
+			break;
+		}
 
 		if ( ! $post_type->map_meta_cap ) {
 			$caps[] = $post_type->cap->$cap;
@@ -1120,6 +1130,11 @@
 		}
 
 		$post_type = get_post_type_object( $post->post_type );
+		
+		if ( is_null( $post_type ) ) {
+			$caps[] = 'do_not_allow';
+			break;
+		}
 
 		if ( ! $post_type->map_meta_cap ) {
 			$caps[] = $post_type->cap->$cap;
@@ -1161,6 +1176,12 @@
 	case 'add_post_meta':
 		$post = get_post( $args[0] );
 		$post_type_object = get_post_type_object( $post->post_type );
+
+		if ( is_null( $post_type_object ) ) {
+			$caps[] = 'do_not_allow';
+			break;
+		}
+		
 		$caps = map_meta_cap( $post_type_object->cap->edit_post, $user_id, $post->ID );
 
 		$meta_key = isset( $args[ 1 ] ) ? $args[ 1 ] : false;
@@ -1178,6 +1199,11 @@
 		$post = get_post( $comment->comment_post_ID );
 		$post_type_object = get_post_type_object( $post->post_type );
 
+		if ( is_null( $post_type_object ) ) {
+			$caps[] = 'do_not_allow';
+			break;
+		}
+
 		$caps = map_meta_cap( $post_type_object->cap->edit_post, $user_id, $post->ID );
 		break;
 	case 'unfiltered_upload':