diff --git a/lib/reporter.js b/lib/reporter.js
index a301560..bbcd61f 100644
--- a/lib/reporter.js
+++ b/lib/reporter.js
@@ -63,11 +63,12 @@ var CoverageReporter = function(rootConfig, emitter, helper, logger) {
     }
   }
 
-  this.onRunStart = function(browsers) {
-    collectors = {};
-    browsers.forEach(function(browser) {
-      collectors[browser.id] = new istanbul.Collector();
-    });
+  this.onRunStart = function() {
+    collectors = Object.create(null);
+  };
+
+  this.onBrowserStart = function(browser) {
+    collectors[browser.id] = new istanbul.Collector();
   };
 
   this.onBrowserComplete = function(browser, result) {
diff --git a/test/reporter.spec.coffee b/test/reporter.spec.coffee
index 7447f05..9f7aaa0 100644
--- a/test/reporter.spec.coffee
+++ b/test/reporter.spec.coffee
@@ -118,7 +118,8 @@ describe 'reporter', ->
       fakeOpera = makeBrowser 'bbb', 'Opera/9.80 Mac OS X Version/12.00', browsers, emitter
       browsers.add fakeChrome
       browsers.add fakeOpera
-      reporter.onRunStart browsers
+      reporter.onRunStart()
+      browsers.forEach (b) -> reporter.onBrowserStart b
       mockFs.writeFile.reset()
       mockMkdir.reset()