diff --git a/README.md b/README.md
index 746ee3b8..a3efbb1c 100644
--- a/README.md
+++ b/README.md
@@ -132,7 +132,7 @@ expect(actionThree(3)).to.deep.equal({
 });
 ```
 
-### `handleAction(type, reducer | reducerMap, ?defaultState)`
+### `handleAction(type, reducer | reducerMap, defaultState)`
 
 ```js
 import { handleAction } from 'redux-actions';
@@ -148,14 +148,14 @@ Otherwise, you can specify separate reducers for `next()` and `throw()`. This AP
 handleAction('FETCH_DATA', {
   next(state, action) {...},
   throw(state, action) {...}
-});
+}, defaultState);
 ```
 
 If either `next()` or `throw()` are `undefined` or `null`, then the identity function is used for that reducer.
 
-The optional third parameter specifies a default or initial state, which is used when `undefined` is passed to the reducer.
+The third parameter `defaultState` is required, and is used when `undefined` is passed to the reducer.
 
-### `handleActions(reducerMap, ?defaultState)`
+### `handleActions(reducerMap, defaultState)`
 
 ```js
 import { handleActions } from 'redux-actions';
@@ -163,7 +163,7 @@ import { handleActions } from 'redux-actions';
 
 Creates multiple reducers using `handleAction()` and combines them into a single reducer that handles multiple actions. Accepts a map where the keys are passed as the first parameter to `handleAction()` (the action type), and the values are passed as the second parameter (either a reducer or reducer map).
 
-The optional second parameter specifies a default or initial state, which is used when `undefined` is passed to the reducer.
+The second parameter `defaultState` is required, and is used when `undefined` is passed to the reducer.
 
 (Internally, `handleActions()` works by applying multiple reducers in sequence using [reduce-reducers](https://github.com/acdlite/reduce-reducers).)
 
diff --git a/src/__tests__/handleAction-test.js b/src/__tests__/handleAction-test.js
index ff8f5018..a62d2e09 100644
--- a/src/__tests__/handleAction-test.js
+++ b/src/__tests__/handleAction-test.js
@@ -5,11 +5,21 @@ import { handleAction, createAction, createActions, combineActions } from '../';
 describe('handleAction()', () => {
   const type = 'TYPE';
   const prevState = { counter: 3 };
+  const defaultState = { counter: 0 };
 
   describe('single handler form', () => {
+    it('should throw an error if defaultState is not specified', () => {
+      expect(() => {
+        handleAction(type, undefined);
+      }).to.throw(
+        Error,
+        'defaultState for reducer handling TYPE should be defined'
+      );
+    });
+
     describe('resulting reducer', () => {
       it('returns previous state if type does not match', () => {
-        const reducer = handleAction('NOTTYPE', () => null);
+        const reducer = handleAction('NOTTYPE', () => null, defaultState);
         expect(reducer(prevState, { type })).to.equal(prevState);
       });
 
@@ -24,7 +34,7 @@ describe('handleAction()', () => {
       it('accepts single function as handler', () => {
         const reducer = handleAction(type, (state, action) => ({
           counter: state.counter + action.payload
-        }));
+        }), defaultState);
         expect(reducer(prevState, { type, payload: 7 }))
           .to.deep.equal({
             counter: 10
@@ -35,7 +45,7 @@ describe('handleAction()', () => {
         const incrementAction = createAction(type);
         const reducer = handleAction(incrementAction, (state, action) => ({
           counter: state.counter + action.payload
-        }));
+        }), defaultState);
 
         expect(reducer(prevState, incrementAction(7)))
           .to.deep.equal({
@@ -43,7 +53,7 @@ describe('handleAction()', () => {
           });
       });
 
-      it('accepts single function as handler and a default state', () => {
+      it('accepts a default state used when the previous state is undefined', () => {
         const reducer = handleAction(type, (state, action) => ({
           counter: state.counter + action.payload
         }), { counter: 3 });
@@ -59,20 +69,30 @@ describe('handleAction()', () => {
 
         const reducer = handleAction(increment, (state, { payload }) => ({
           counter: state.counter + payload
-        }), { counter: 3 });
+        }), defaultState);
 
         expect(reducer(undefined, increment(7)))
           .to.deep.equal({
-            counter: 10
+            counter: 7
           });
       });
     });
   });
 
   describe('map of handlers form', () => {
+    it('should throw an error if defaultState is not specified', () => {
+      expect(() => {
+        handleAction(type, { next: () => null });
+      })
+      .to.throw(
+        Error,
+        'defaultState for reducer handling TYPE should be defined'
+      );
+    });
+
     describe('resulting reducer', () => {
       it('returns previous state if type does not match', () => {
-        const reducer = handleAction('NOTTYPE', { next: () => null });
+        const reducer = handleAction('NOTTYPE', { next: () => null }, defaultState);
         expect(reducer(prevState, { type })).to.equal(prevState);
       });
 
@@ -81,7 +101,7 @@ describe('handleAction()', () => {
           next: (state, action) => ({
             counter: state.counter + action.payload
           })
-        });
+        }, defaultState);
         expect(reducer(prevState, { type, payload: 7 }))
           .to.deep.equal({
             counter: 10
@@ -93,7 +113,7 @@ describe('handleAction()', () => {
           throw: (state, action) => ({
             counter: state.counter + action.payload
           })
-        });
+        }, defaultState);
 
         expect(reducer(prevState, { type, payload: 7, error: true }))
           .to.deep.equal({
@@ -102,7 +122,7 @@ describe('handleAction()', () => {
       });
 
       it('returns previous state if matching handler is not function', () => {
-        const reducer = handleAction(type, { next: null, error: 123 });
+        const reducer = handleAction(type, { next: null, error: 123 }, defaultState);
         expect(reducer(prevState, { type, payload: 123 })).to.equal(prevState);
         expect(reducer(prevState, { type, payload: 123, error: true }))
           .to.equal(prevState);
@@ -115,7 +135,8 @@ describe('handleAction()', () => {
       const action1 = createAction('ACTION_1');
       const reducer = handleAction(
         combineActions(action1, 'ACTION_2', 'ACTION_3'),
-        (state, { payload }) => ({ ...state, number: state.number + payload })
+        (state, { payload }) => ({ ...state, number: state.number + payload }),
+        defaultState
       );
 
       expect(reducer({ number: 1 }, action1(1))).to.deep.equal({ number: 2 });
@@ -129,7 +150,7 @@ describe('handleAction()', () => {
         next(state, { payload }) {
           return { ...state, number: state.number + payload };
         }
-      });
+      }, defaultState);
 
       expect(reducer({ number: 1 }, action1(1))).to.deep.equal({ number: 2 });
       expect(reducer({ number: 1 }, { type: 'ACTION_2', payload: 2 })).to.deep.equal({ number: 3 });
@@ -146,7 +167,7 @@ describe('handleAction()', () => {
         throw(state) {
           return { ...state, threw: true };
         }
-      });
+      }, defaultState);
       const error = new Error;
 
       expect(reducer({ number: 0 }, action1(error)))
@@ -161,6 +182,7 @@ describe('handleAction()', () => {
       const reducer = handleAction(
         combineActions('ACTION_1', 'ACTION_2'),
         (state, { payload }) => ({ ...state, state: state.number + payload }),
+        defaultState
       );
 
       const state = { number: 0 };
@@ -172,11 +194,11 @@ describe('handleAction()', () => {
       const reducer = handleAction(
         combineActions('INCREMENT', 'DECREMENT'),
         (state, { payload }) => ({ ...state, counter: state.counter + payload }),
-        { counter: 10 }
+        defaultState
       );
 
-      expect(reducer(undefined, { type: 'INCREMENT', payload: +1 })).to.deep.equal({ counter: 11 });
-      expect(reducer(undefined, { type: 'DECREMENT', payload: -1 })).to.deep.equal({ counter: 9 });
+      expect(reducer(undefined, { type: 'INCREMENT', payload: +1 })).to.deep.equal({ counter: +1 });
+      expect(reducer(undefined, { type: 'DECREMENT', payload: -1 })).to.deep.equal({ counter: -1 });
     });
 
     it('should handle combined actions with symbols', () => {
@@ -185,7 +207,8 @@ describe('handleAction()', () => {
       const action3 = createAction(Symbol('ACTION_3'));
       const reducer = handleAction(
         combineActions(action1, action2, action3),
-        (state, { payload }) => ({ ...state, number: state.number + payload })
+        (state, { payload }) => ({ ...state, number: state.number + payload }),
+        defaultState
       );
 
       expect(reducer({ number: 0 }, action1(1)))
@@ -199,7 +222,7 @@ describe('handleAction()', () => {
 
   describe('with invalid actions', () => {
     it('should throw a descriptive error when the action object is missing', () => {
-      const reducer = handleAction(createAction('ACTION_1'), identity);
+      const reducer = handleAction(createAction('ACTION_1'), identity, {});
       expect(
         () => reducer(undefined)
       ).to.throw(
@@ -209,7 +232,7 @@ describe('handleAction()', () => {
     });
 
     it('should throw a descriptive error when the action type is missing', () => {
-      const reducer = handleAction(createAction('ACTION_1'), identity);
+      const reducer = handleAction(createAction('ACTION_1'), identity, {});
       expect(
         () => reducer(undefined, {})
       ).to.throw(
@@ -219,7 +242,7 @@ describe('handleAction()', () => {
     });
 
     it('should throw a descriptive error when the action type is not a string or symbol', () => {
-      const reducer = handleAction(createAction('ACTION_1'), identity);
+      const reducer = handleAction(createAction('ACTION_1'), identity, {});
       expect(
         () => reducer(undefined, { type: false })
       ).to.throw(
diff --git a/src/__tests__/handleActions-test.js b/src/__tests__/handleActions-test.js
index 6335f2d2..3c6a6e08 100644
--- a/src/__tests__/handleActions-test.js
+++ b/src/__tests__/handleActions-test.js
@@ -2,6 +2,43 @@ import { expect } from 'chai';
 import { handleActions, createAction, createActions, combineActions } from '../';
 
 describe('handleActions', () => {
+  const defaultState = { counter: 0 };
+
+  it('should throw an error when defaultState is not defined', () => {
+    expect(() => {
+      handleActions({
+        INCREMENT: ({ counter }, { payload: amount }) => ({
+          counter: counter + amount
+        }),
+
+        DECREMENT: ({ counter }, { payload: amount }) => ({
+          counter: counter - amount
+        })
+      });
+    }).to.throw(
+      Error,
+      'defaultState for reducer handling INCREMENT should be defined'
+    );
+  });
+
+  it('should throw an error when defaultState is not defined for combinedActions', () => {
+    expect(() => {
+      handleActions({
+        [
+          combineActions(
+            'INCREMENT',
+            'DECREMENT'
+          )
+        ]: ({ counter }, { type, payload: amount }) => ({
+          counter: counter + (type === 'INCREMENT' ? +1 : -1) * amount
+        })
+      });
+    }).to.throw(
+      Error,
+      'defaultState for reducer handling INCREMENT, DECREMENT should be defined'
+    );
+  });
+
   it('create a single handler from a map of multiple action handlers', () => {
     const reducer = handleActions({
       INCREMENT: ({ counter }, { payload: amount }) => ({
@@ -11,7 +48,7 @@ describe('handleActions', () => {
       DECREMENT: ({ counter }, { payload: amount }) => ({
         counter: counter - amount
       })
-    });
+    }, defaultState);
 
     expect(reducer({ counter: 3 }, { type: 'INCREMENT', payload: 7 }))
       .to.deep.equal({
@@ -30,7 +67,7 @@ describe('handleActions', () => {
       [INCREMENT]: ({ counter }, { payload: amount }) => ({
         counter: counter + amount
       })
-    });
+    }, defaultState);
 
     expect(reducer({ counter: 3 }, { type: INCREMENT, payload: 7 }))
       .to.deep.equal({
@@ -38,7 +75,7 @@ describe('handleActions', () => {
       });
   });
 
-  it('accepts a default state as the second parameter', () => {
+  it('accepts a default state used when previous state is undefined', () => {
     const reducer = handleActions({
       INCREMENT: ({ counter }, { payload: amount }) => ({
         counter: counter + amount
@@ -61,7 +98,7 @@ describe('handleActions', () => {
       [incrementAction]: ({ counter }, { payload: amount }) => ({
         counter: counter + amount
       })
-    });
+    }, defaultState);
 
     expect(reducer({ counter: 3 }, incrementAction(7)))
       .to.deep.equal({
@@ -81,12 +118,12 @@ describe('handleActions', () => {
       [combineActions(increment, decrement)](state, { payload: { amount } }) {
         return { ...state, counter: state.counter + amount };
       }
-    }, initialState);
+    }, defaultState);
 
     expect(reducer(initialState, increment(5))).to.deep.equal({ counter: 15 });
     expect(reducer(initialState, decrement(5))).to.deep.equal({ counter: 5 });
     expect(reducer(initialState, { type: 'NOT_TYPE', payload: 1000 })).to.equal(initialState);
-    expect(reducer(undefined, increment(5))).to.deep.equal({ counter: 15 });
+    expect(reducer(undefined, increment(5))).to.deep.equal({ counter: 5 });
   });
 
   it('should accept combined actions as action types in the next/throw form', () => {
@@ -107,14 +144,14 @@ describe('handleActions', () => {
           return { ...state, counter: 0 };
         }
       }
-    }, initialState);
+    }, defaultState);
     const error = new Error;
 
     // non-errors
     expect(reducer(initialState, increment(5))).to.deep.equal({ counter: 15 });
     expect(reducer(initialState, decrement(5))).to.deep.equal({ counter: 5 });
     expect(reducer(initialState, { type: 'NOT_TYPE', payload: 1000 })).to.equal(initialState);
-    expect(reducer(undefined, increment(5))).to.deep.equal({ counter: 15 });
+    expect(reducer(undefined, increment(5))).to.deep.equal({ counter: 5 });
 
     // errors
     expect(
@@ -136,7 +173,7 @@ describe('handleActions', () => {
       [decrement]: ({ counter }, { payload }) => ({
         counter: counter - payload
       })
-    });
+    }, defaultState);
 
     expect(reducer({ counter: 3 }, increment(2)))
       .to.deep.equal({
diff --git a/src/handleAction.js b/src/handleAction.js
index ed9f56af..4c5231ba 100644
--- a/src/handleAction.js
+++ b/src/handleAction.js
@@ -1,6 +1,7 @@
 import isFunction from 'lodash/isFunction';
 import identity from 'lodash/identity';
 import isNil from 'lodash/isNil';
+import isUndefined from 'lodash/isUndefined';
 import includes from 'lodash/includes';
 import invariant from 'invariant';
 import { isFSA } from 'flux-standard-action';
@@ -8,6 +9,10 @@ import { ACTION_TYPE_DELIMITER } from './combineActions';
 
 export default function handleAction(actionType, reducers, defaultState) {
   const actionTypes = actionType.toString().split(ACTION_TYPE_DELIMITER);
+  invariant(
+    !isUndefined(defaultState),
+    `defaultState for reducer handling ${actionTypes.join(', ')} should be defined`
+  );
 
   const [nextReducer, throwReducer] = isFunction(reducers)
     ? [reducers, reducers]
diff --git a/src/handleActions.js b/src/handleActions.js
index 69589d02..69d7a83d 100644
--- a/src/handleActions.js
+++ b/src/handleActions.js
@@ -3,8 +3,13 @@ import ownKeys from './ownKeys';
 import reduceReducers from 'reduce-reducers';
 
 export default function handleActions(handlers, defaultState) {
-  const reducers = ownKeys(handlers).map(type => handleAction(type, handlers[type]));
+  const reducers = ownKeys(handlers).map(type =>
+    handleAction(
+      type,
+      handlers[type],
+      defaultState
+    )
+  );
   const reducer = reduceReducers(...reducers);
-
-  return (state = defaultState, action) => reducer(state, action);
+  return (state, action) => reducer(state, action);
 }