diff --git a/src/actions/tree.ts b/src/actions/tree.ts
index f9708bf40..34967f0e4 100644
--- a/src/actions/tree.ts
+++ b/src/actions/tree.ts
@@ -16,8 +16,10 @@ import { Metadata } from "../metadata";
 import { AppDispatch, RootState } from "../store";
 import { ReduxNode, TreeState } from "../reducers/tree/types";
 
+type RootIndex = number | undefined
+
 /** [root idx tree1, root idx tree2] */
-export type Root = [number | undefined, number | undefined]
+export type Root = [RootIndex, RootIndex]
 
 /** A function to be handled by redux (thunk) */
 type ThunkFunction = (dispatch: AppDispatch, getState: () => RootState) => void
@@ -31,7 +33,7 @@ type ThunkFunction = (dispatch: AppDispatch, getState: () => RootState) => void
  */
 export const applyInViewNodesToTree = (
   /** index of displayed root node */
-  idx: number | undefined,
+  idx: RootIndex,
 
   tree: TreeState,
 ): number => {