diff --git a/lib/gui_qt.py b/lib/gui_qt.py
index 47e419cd4..5e7b9da56 100644
--- a/lib/gui_qt.py
+++ b/lib/gui_qt.py
@@ -1298,7 +1298,7 @@ class ElectrumWindow(QMainWindow):
                     QMessageBox.warning(self, _('Error'), _('Invalid value'), _('OK'))
                     
             cfg = SimpleConfig()
-            cfg.config["gui"] = str(gui.currentText()).lower()
+            cfg.set_key("gui", str(gui.currentText()).lower())
             cfg.save_config()
 
         self.set_expert_mode(cb.isChecked())