--- a/browser/extensions/clicktoplay-rollout/bootstrap.js
+++ b/browser/extensions/clicktoplay-rollout/bootstrap.js
@@ -41,17 +41,17 @@ function startup() {
 }
 
 function defineCohort() {
   let updateChannel = UpdateUtils.getUpdateChannel(false);
   if (!(updateChannel in TEST_THRESHOLD)) {
     return;
   }
 
-  let cohort = Services.prefs.getStringPref(PREF_COHORT_NAME);
+  let cohort = Services.prefs.getStringPref(PREF_COHORT_NAME, undefined);
 
   if (!cohort) {
     // The cohort has not been defined yet: this is the first
     // time that we're running. Let's see if the user has
     // a non-default setting to avoid changing it.
     let currentPluginState = Services.prefs.getIntPref(PREF_FLASH_STATE);
     switch (currentPluginState) {
       case Ci.nsIPluginTag.STATE_CLICKTOPLAY: