diff --git a/Libraries/CustomComponents/Navigator/Navigation/NavigationRouteStack.js b/Libraries/CustomComponents/Navigator/Navigation/NavigationRouteStack.js
index fd0d93222a..4862851774 100644
--- a/Libraries/CustomComponents/Navigator/Navigation/NavigationRouteStack.js
+++ b/Libraries/CustomComponents/Navigator/Navigation/NavigationRouteStack.js
@@ -117,7 +117,7 @@ class RouteStack {
     return this._routeNodes.findIndex(finder, this);
   }
 
-  slice(begin: ?number, end: ?number): RouteStack {
+  slice(begin?: number, end?: number): RouteStack {
     var routeNodes = this._routeNodes.slice(begin, end);
     var index = Math.min(this._index, routeNodes.size - 1);
     return this._update(index, routeNodes);