diff --git a/flag.inc b/flag.inc
index 56e5b11..825266b 100644
--- a/flag.inc
+++ b/flag.inc
@@ -863,7 +863,9 @@ class flag_node extends flag_flag {
   }
 
   function get_content_id($node) {
-    return $node->nid;
+    if ($node) {
+      return $node->nid;
+    }
   }
 
   /**