first argument must be fn or undefined

Author: echenley

src/instrument.js

@@ -347,9 +347,7 @@ function unliftStore(liftedStore, liftReducer) {
 /**
  * Redux instrumentation store enhancer.
  */
-export default function instrument(monitorReducer, options = {}) {
-  if (!monitorReducer) { monitorReducer = () => null; }
-
+export default function instrument(monitorReducer = () => null, options = {}) {
   return createStore => (reducer, initialState, enhancer) => {
 
     function liftReducer(r) {

test/instrument.spec.js

@@ -300,7 +300,7 @@ describe('instrument', () => {
     let configuredLiftedStore;
 
     beforeEach(() => {
-      configuredStore = createStore(counter, instrument(null, { maxAge: 2 }));
+      configuredStore = createStore(counter, instrument(undefined, { maxAge: 2 }));
       configuredLiftedStore = configuredStore.liftedStore;
     });