diff --git a/tutorial/12-flow/src/client/reducers/dog-reducer.js b/tutorial/12-flow/src/client/reducers/dog-reducer.js
index 5af5ba65fb53b5efb8dbca26532d6ecd99bee6c1..fad3f094ad0cc5515b84f50449b45108cf488401 100644
--- a/tutorial/12-flow/src/client/reducers/dog-reducer.js
+++ b/tutorial/12-flow/src/client/reducers/dog-reducer.js
@@ -7,7 +7,7 @@ const initialState = Immutable.Map({
   hasBarked: false,
 });
 
-const dogReducer = (state: Object = initialState, action: Object) => {
+const dogReducer = (state: Object = initialState, action: {type: string, [key: string]: any}) => {
   switch (action.type) {
     case MAKE_BARK:
       return state.set('hasBarked', action.payload);