diff --git a/calendar/lightning/content/agenda-tree.js b/calendar/lightning/content/agenda-tree.js
index fc76ba0734d9..9b107528010d 100644
--- a/calendar/lightning/content/agenda-tree.js
+++ b/calendar/lightning/content/agenda-tree.js
@@ -199,7 +199,7 @@ function getParentIndex(row)
         if (this.events[i] instanceof Synthetic)
             return i;
         i--;
-    } while (i != 0);
+    } while (i != -1);
     throw "no parent for row " + row + "?";
 };