diff --git a/d3/d3.d.ts b/d3/d3.d.ts
index 42f94d0c6..f6f3f9d04 100644
--- a/d3/d3.d.ts
+++ b/d3/d3.d.ts
@@ -1463,7 +1463,7 @@ declare module D3 {
             /**
             * decrease lightness by some exponential factor (gamma)
             */
-            darker(k: number): Color;
+            darker(k?: number): Color;
             /**
             * convert the color to a string.
             */