diff --git a/d3/d3.d.ts b/d3/d3.d.ts
index 0b049726bc..42f94d0c66 100644
--- a/d3/d3.d.ts
+++ b/d3/d3.d.ts
@@ -811,7 +811,7 @@ declare module D3 {
 
     export interface EnterSelection {
         append: (name: string) => Selection;
-        insert: (name: string, before: string) => Selection;
+        insert: (name: string, before?: string) => Selection;
         select: (selector: string) => Selection;
         empty: () => boolean;
         node: () => Element;