diff --git a/src/combineReducers.js b/src/combineReducers.js
index 4929654193..15f42a7d52 100644
--- a/src/combineReducers.js
+++ b/src/combineReducers.js
@@ -57,19 +57,17 @@ function assertReducerSanity(reducers) {
     if (typeof initialState === 'undefined') {
       throw new Error(
         `Reducer "${key}" returned undefined during initialization. ` +
-        `If the state passed to the reducer is undefined, you must ` +
-        `explicitly return the initial state. The initial state may ` +
-        `not be undefined.`
+        `Reducers should never return undefined. Make sure this reducer ` +
+        `has a catch-all clause for unknown action types and that it returns a ` +
+        `default initial state if the state passed to it is undefined.`
       )
     }
 
-    var type = '@@redux/PROBE_UNKNOWN_ACTION_' + Math.random().toString(36).substring(7).split('').join('.')
-    if (typeof reducer(undefined, { type }) === 'undefined') {
+    var randomState = Math.random().toString(36).substring(7).split('').join('.')
+    if (reducer(randomState, { type: ActionTypes.INIT }) !== randomState) {
       throw new Error(
-        `Reducer "${key}" returned undefined when probed with a random type. ` +
-        `Don't try to handle ${ActionTypes.INIT} or other actions in "redux/*" ` +
-        `namespace. They are considered private. Instead, you must return the ` +
-        `current state for any unknown actions, unless it is undefined, ` +
+        `Reducer "${key}" did not return the current state when probed with a random type. ` +
+        `You must return the current state for any unknown actions, unless it is undefined, ` +
         `in which case you must return the initial state, regardless of the ` +
         `action type. The initial state may not be undefined.`
       )
diff --git a/src/createStore.js b/src/createStore.js
index b46ff9554b..86e425fd31 100644
--- a/src/createStore.js
+++ b/src/createStore.js
@@ -7,7 +7,7 @@ import isPlainObject from './utils/isPlainObject'
  * Do not reference these action types directly in your code.
  */
 export var ActionTypes = {
-  INIT: '@@redux/INIT'
+  INIT: '@@redux/INIT_' + Math.random().toString(36).substring(7).split('').join('.')
 }
 
 /**
diff --git a/test/combineReducers.spec.js b/test/combineReducers.spec.js
index a2b00857d0..e3f4aac010 100644
--- a/test/combineReducers.spec.js
+++ b/test/combineReducers.spec.js
@@ -1,6 +1,6 @@
 import expect from 'expect'
 import { combineReducers } from '../src'
-import createStore, { ActionTypes } from '../src/createStore'
+import createStore from '../src/createStore'
 
 describe('Utils', () => {
   describe('combineReducers', () => {
@@ -84,6 +84,25 @@ describe('Utils', () => {
       )
     })
 
+    it('throws an error if reducer does not have a catch-all clause for unknown action types', () => {
+      const reducer = combineReducers({
+        counter(state = 0, action) {
+          switch (action.type) {
+            case 'increment':
+              return state + 1
+            case 'decrement':
+              return state - 1
+            case undefined:
+              return state
+          }
+        }
+      })
+
+      expect(() => reducer()).toThrow(
+        /"counter".*initialization/
+      )
+    })
+
     it('catches error thrown in reducer when initializing and re-throw', () => {
       const reducer = combineReducers({
         throwingReducer() {
@@ -151,7 +170,7 @@ describe('Utils', () => {
       expect(reducer(initialState, { type: 'increment' })).toNotBe(initialState)
     })
 
-    it('throws an error on first call if a reducer attempts to handle a private action', () => {
+    it('throws an error if reducer does not return current state for unknown action types', () => {
       const reducer = combineReducers({
         counter(state, action) {
           switch (action.type) {
@@ -159,16 +178,14 @@ describe('Utils', () => {
               return state + 1
             case 'decrement':
               return state - 1
-            // Never do this in your code:
-            case ActionTypes.INIT:
-              return 0
             default:
-              return undefined
+              return 0
           }
         }
       })
+
       expect(() => reducer()).toThrow(
-        /"counter".*private/
+        /"counter".*probed/
       )
     })