diff --git a/toolkit/content/widgets/preferences.xml b/toolkit/content/widgets/preferences.xml
index 489e64ea951..cb02e88653b 100644
--- a/toolkit/content/widgets/preferences.xml
+++ b/toolkit/content/widgets/preferences.xml
@@ -88,7 +88,8 @@
         // In non-instant apply mode, we must try and use the last saved state
         // from any previous opens of a child dialog instead of the value from
         // preferences, to pick up any edits a user may have made. 
-        if (document.documentElement.type == "child" && !this.instantApply) {
+        if (document.documentElement.type == "child" && 
+            !this.instantApply && window.opener) {
           var pdoc = window.opener.document;
           var lastStatePref = pdoc.getElementById(window.location.href + "#" + this.id);
           this._setValue(lastStatePref ? lastStatePref.value 
@@ -758,7 +759,7 @@
         if (!this._fireEvent("beforeaccept", this)) 
           return;
         
-        if (this.type == "child") {
+        if (this.type == "child" && window.opener) {
           var psvc = Components.classes["@mozilla.org/preferences-service;1"]
                                .getService(Components.interfaces.nsIPrefBranch);
           var instantApply = psvc.getBoolPref("browser.preferences.instantApply");