--- a/dom/system/NetworkGeolocationProvider.js
+++ b/dom/system/NetworkGeolocationProvider.js
@@ -410,18 +410,18 @@ WifiGeoPositionProvider.prototype = {
     LOG("getMobileInfo called");
     try {
       let radioService = Cc["@mozilla.org/ril;1"]
                     .getService(Ci.nsIRadioInterfaceLayer);
       let service = Cc["@mozilla.org/mobileconnection/mobileconnectionservice;1"]
                     .getService(Ci.nsIMobileConnectionService);
 
       let result = [];
-      for (let i = 0; i < service.length; i++) {
-        LOG("Looking for SIM in slot:" + i + " of " + service.length);
+      for (let i = 0; i < service.numItems; i++) {
+        LOG("Looking for SIM in slot:" + i + " of " + service.numItems);
         let connection = service.getItemByServiceId(i);
         let voice = connection && connection.voice;
         let cell = voice && voice.cell;
         let type = voice && voice.type;
         let network = voice && voice.network;
 
         if (network && cell && type) {
           let radioTechFamily;