diff --git a/addon-sdk/source/lib/sdk/tabs/utils.js b/addon-sdk/source/lib/sdk/tabs/utils.js
index eae3d41fe647..9edf35314976 100644
--- a/addon-sdk/source/lib/sdk/tabs/utils.js
+++ b/addon-sdk/source/lib/sdk/tabs/utils.js
@@ -207,7 +207,7 @@ function getTabId(tab) {
   if (tab.browser) // fennec
     return tab.id
 
-  return String.split(tab.linkedPanel, 'panel').pop();
+  return String(tab.linkedPanel).split('panel').pop();
 }
 exports.getTabId = getTabId;
 
diff --git a/addon-sdk/source/lib/sdk/url.js b/addon-sdk/source/lib/sdk/url.js
index ae16ac4a8cad..cfce03354cac 100644
--- a/addon-sdk/source/lib/sdk/url.js
+++ b/addon-sdk/source/lib/sdk/url.js
@@ -336,7 +336,7 @@ var isValidURI = exports.isValidURI = function (uri) {
 }
 
 function isLocalURL(url) {
-  if (String.indexOf(url, './') === 0)
+  if (String(url).indexOf('./') === 0)
     return true;
 
   try {