diff --git a/skimage/viewer/qt.py b/skimage/viewer/qt.py
index 0f6f5616..8f6768a5 100644
--- a/skimage/viewer/qt.py
+++ b/skimage/viewer/qt.py
@@ -1,10 +1,13 @@
+_qt_version = None
+has_qt = True
+
 try:
-    from matplotlib.backends.qt_compat import QtGui, QtCore, QtWidgets, QT_RC_MAJOR_VERSION as has_qt
+    from matplotlib.backends.qt_compat import QtGui, QtCore, QtWidgets, QT_RC_MAJOR_VERSION as _qt_version
 except ImportError:
     try:
         from matplotlib.backends.qt4_compat import QtGui, QtCore
         QtWidgets = QtGui
-        has_qt = 4
+        _qt_version = 4
     except ImportError:
         # Mock objects
         class QtGui_cls(object):
@@ -27,14 +30,13 @@ except ImportError:
 
         has_qt = False
 
-if has_qt == 5:
+if _qt_version == 5:
     from matplotlib.backends.backend_qt5 import FigureManagerQT
     from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg
-elif has_qt == 4:
+elif _qt_version == 4:
     from matplotlib.backends.backend_qt4 import FigureManagerQT
     from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg
 else:
-    assert not has_qt, 'Unsupported Qt version {0}'.format(has_qt)
     FigureManagerQT = object
     FigureCanvasQTAgg = object