diff --git a/src/calibre/gui2/dialogs/config.py b/src/calibre/gui2/dialogs/config.py
index 2b093a45b5..1d5fad960e 100644
--- a/src/calibre/gui2/dialogs/config.py
+++ b/src/calibre/gui2/dialogs/config.py
@@ -180,12 +180,12 @@ class ConfigDialog(QDialog, Ui_Dialog):
         self.toolbar_button_size.setCurrentIndex(0 if icons == self.ICON_SIZES[0] else 1 if icons == self.ICON_SIZES[1] else 2)
         self.show_toolbar_text.setChecked(config['show_text_in_toolbar'])
 
-        book_exts = sorted(BOOK_EXTENSIONS)
-        for ext in book_exts:
+        self.book_exts = sorted(BOOK_EXTENSIONS)
+        for ext in self.book_exts:
             self.single_format.addItem(ext.upper(), QVariant(ext))
         
         single_format = config['save_to_disk_single_format']
-        self.single_format.setCurrentIndex(book_exts.index(single_format))
+        self.single_format.setCurrentIndex(self.book_exts.index(single_format))
         self.cover_browse.setValue(config['cover_flow_queue_length'])
         self.systray_notifications.setChecked(not config['disable_tray_notification'])
         from calibre.translations.compiled import translations
@@ -204,7 +204,7 @@ class ConfigDialog(QDialog, Ui_Dialog):
         self.pdf_metadata.setChecked(prefs['read_file_metadata'])
         
         added_html = False
-        for ext in book_exts:
+        for ext in self.book_exts:
             ext = ext.lower()
             ext = re.sub(r'(x{0,1})htm(l{0,1})', 'html', ext)
             if ext == 'lrf' or is_supported('book.'+ext):
@@ -402,7 +402,7 @@ class ConfigDialog(QDialog, Ui_Dialog):
         p = {0:'normal', 1:'high', 2:'low'}[self.priority.currentIndex()]
         prefs['worker_process_priority'] = p
         prefs['read_file_metadata'] = bool(self.pdf_metadata.isChecked())
-        config['save_to_disk_single_format'] = BOOK_EXTENSIONS[self.single_format.currentIndex()]
+        config['save_to_disk_single_format'] = self.book_exts[self.single_format.currentIndex()]
         config['cover_flow_queue_length'] = self.cover_browse.value()
         prefs['language'] = str(self.language.itemData(self.language.currentIndex()).toString())
         config['systray_icon'] = self.systray_icon.checkState() == Qt.Checked