diff --git a/docs/core/howto/choosing-reactor.rst b/docs/core/howto/choosing-reactor.rst
index 3d5aa19..f6bb84b 100644
--- a/docs/core/howto/choosing-reactor.rst
+++ b/docs/core/howto/choosing-reactor.rst
@@ -448,6 +448,7 @@ done before running the reactor:
     from twisted.internet import gtk3reactor
     gtk3reactor.install()
     
+    gi.require_version('Gtk', '3.0')
     from gi.repository import Gtk
     app = Gtk.Application(...)
     
diff --git a/src/twisted/internet/gireactor.py b/src/twisted/internet/gireactor.py
index 92596db..c061b48 100644
--- a/src/twisted/internet/gireactor.py
+++ b/src/twisted/internet/gireactor.py
@@ -68,6 +68,7 @@ class GIReactor(_glibbase.GlibReactorBase):
     def __init__(self, useGtk=False):
         _gtk = None
         if useGtk is True:
+            gi.require_version('Gtk', '3.0')
             from gi.repository import Gtk as _gtk
 
         _glibbase.GlibReactorBase.__init__(self, GLib, _gtk, useGtk=useGtk)
@@ -112,6 +113,7 @@ class PortableGIReactor(_glibbase.PortableGlibReactorBase):
     def __init__(self, useGtk=False):
         _gtk = None
         if useGtk is True:
+            gi.require_version('Gtk', '3.0')
             from gi.repository import Gtk as _gtk
 
         _glibbase.PortableGlibReactorBase.__init__(self, GLib, _gtk, useGtk=useGtk)
diff --git a/src/twisted/internet/test/test_gireactor.py b/src/twisted/internet/test/test_gireactor.py
index d15a926..600472e 100644
--- a/src/twisted/internet/test/test_gireactor.py
+++ b/src/twisted/internet/test/test_gireactor.py
@@ -25,6 +25,7 @@ else:
         gtk3reactor = None
     else:
         gtk3reactor = _gtk3reactor
+        gi.require_version('Gtk', '3.0')
         from gi.repository import Gtk
 
 from twisted.internet.error import ReactorAlreadyRunning