diff --git a/d3/d3.d.ts b/d3/d3.d.ts
index be4032921..099b6e72b 100644
--- a/d3/d3.d.ts
+++ b/d3/d3.d.ts
@@ -1817,7 +1817,7 @@ declare module D3 {
                 (): number;
                 (value: number): Axis;
             }
-            tickFormat(formatter: (value: any) => string): Axis;
+            tickFormat(formatter: (value: any, index?: number) => string): Axis;
             nice(count?: number): Axis;
         }