diff --git a/mithril.d.ts b/mithril.d.ts
index 1a1e1562..784ea6bb 100644
--- a/mithril.d.ts
+++ b/mithril.d.ts
@@ -68,7 +68,7 @@ declare module _mithril {
 
 	// Configuration function for an element
 	interface MithrilElementConfig {
-		(element: Element, isInitialized: boolean, context?: any, vdom: MithrilVirtualElement): void;
+		(element: Element, isInitialized: boolean, context?: any, vdom?: MithrilVirtualElement): void;
 	}
 
 	// Attributes on a virtual element