--- a/testing/marionette/puppeteer/firefox/firefox_puppeteer/api/prefs.py
+++ b/testing/marionette/puppeteer/firefox/firefox_puppeteer/api/prefs.py
@@ -121,17 +121,19 @@ class Preferences(BaseLib):
         with self.marionette.using_context('chrome'):
             # Backup original value only once
             if pref_name not in self.archive:
                 self.archive[pref_name] = self.get_pref(pref_name)
 
             retval = self.marionette.execute_script("""
               Components.utils.import("resource://gre/modules/Services.jsm");
 
-              let [pref_name, value, default_branch] = arguments;
+              let pref_name = arguments[0];
+              let value = arguments[1];
+              let default_branch = arguments[2];
 
               let prefBranch;
               if (default_branch) {
                 prefBranch = Services.prefs.getDefaultBranch("");
               }
               else {
                 prefBranch = Services.prefs;
               }