diff --git a/skimage/viewer/qt/QtCore.py b/skimage/viewer/qt/QtCore.py
index 897b8760..e0def041 100644
--- a/skimage/viewer/qt/QtCore.py
+++ b/skimage/viewer/qt/QtCore.py
@@ -7,3 +7,4 @@ elif qt_api == 'pyqt':
 else:
     # Mock objects
     Qt = None
+    pyqtSignal = None