diff --git a/src/action-reducer.js b/src/action-reducer.js
index 5006abe..1e67bd6 100644
--- a/src/action-reducer.js
+++ b/src/action-reducer.js
@@ -133,7 +133,7 @@ export default function reducer(state, action, mapImplementation = Object) {
         //       top-level component?
         const { path, func } = r;
         const newState = func(mut.getIn(path), action);
-        if (newState === undefined) {
+        if (newState === undefined && mut.getIn(path) !== undefined) {
           throw new Error(`Your custom UI reducer at path ${path.join('.')} must return some state`);
         }
         mut.setIn(path, newState);