diff --git a/colourterm/mainwindow.py b/colourterm/mainwindow.py
index 0c8978b..ced1fb5 100644
--- a/colourterm/mainwindow.py
+++ b/colourterm/mainwindow.py
@@ -238,9 +238,17 @@ class MainWindow(QtGui.QMainWindow, UiMainWindow):
                 self.device_closed = True
                 self.device.close()
             try:
-                settings.setValue(u'previous-port', self.connect_dialog.get_port())
+                port = self.connect_dialog.get_port()
+                settings.setValue(u'previous-port', port)
+                if isinstance(port, basestring) and port.startswith('COM'):
+                    try:
+                        # On Windows ports are 0-based, so strip the COM and subtract 1 from the port number
+                        port = int(port[3:]) - 1
+                    except (TypeError, ValueError):
+                        QtGui.QMessageBox.critical(self, 'Error opening port', 'Error: Port is not valid')
+                        return
                 self.device = Serial(
-                    port=self.connect_dialog.get_port(),
+                    port=port,
                     baudrate=self.connect_dialog.get_baud(),
                     bytesize=self.connect_dialog.get_data_bits(),
                     parity=self.connect_dialog.get_parity(),