How to make a Gtk.ComboBoxText's entry editable in Glade

I'm using Glade to write a PyGI app, and I've come across a problem whereby I've got a Gtk.ComboBoxText with a Gtk.Entry, but I cannot make the entry editable for some reason. I.e. when the UI is loaded, there is no way to input text in it.

I've looked at all the entry's properties and also those of the comboboxtext parent, but I could not find anything obvious that I'm missing. Oddly enough, if I create it with code only (i.e. with no Glade .ui files), that seems to work and the entry is then editable.

This code does not work

Code:

#!/usr/bin/env python

from gi.repository import Gtk

class Combo:
    def __init__(self):
        builder = Gtk.Builder()
        builder.add_from_file('combo.ui')
        window = builder.get_object('window1')
        window.connect('destroy', lambda w: Gtk.main_quit())
        window.show_all()

Combo()
Gtk.main()

Glade file:

<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <!-- interface-requires gtk+ 3.0 -->
  <object class="GtkWindow" id="window1">
    <property name="can_focus">False</property>
    <child>
      <object class="GtkComboBoxText" id="comboboxtext1">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <property name="has_entry">True</property>
        <property name="entry_text_column">0</property>
        <property name="id_column">1</property>
        <child internal-child="entry">
          <object class="GtkEntry" id="comboboxtext-entry">
            <property name="can_focus">False</property>
          </object>
        </child>
      </object>
    </child>
  </object>
</interface>

This code works

#!/usr/bin/env python

from gi.repository import Gtk

class Combo:
    def __init__(self):
        window = Gtk.Window()
        window.set_default_size(200, 200)

        combo = Gtk.ComboBoxText.new_with_entry()
        combo.set_hexpand(True)

        window.connect("destroy", lambda w: Gtk.main_quit())

        window.add(combo)
        window.show_all()

Combo()
Gtk.main()

Any pointers on how to make the text entry editable in the Glade code?