diff --git a/lib/matplotlib/backends/backend_qt4.py b/lib/matplotlib/backends/backend_qt4.py
index 623b245ef7da..99d2813260f4 100644
--- a/lib/matplotlib/backends/backend_qt4.py
+++ b/lib/matplotlib/backends/backend_qt4.py
@@ -122,8 +122,13 @@ class FigureCanvasQT( QtGui.QWidget, FigureCanvasBase ):
                 QtCore.Qt.Key_Alt : 'alt',
                 QtCore.Qt.Key_Return : 'enter'
                }
-    # left 1, middle 2, right 3
-    buttond = {1:1, 2:3, 4:2}
+    # map Qt button codes to MouseEvent's ones:
+    buttond = {QtCore.Qt.LeftButton  : 1,
+               QtCore.Qt.MidButton   : 2,
+               QtCore.Qt.RightButton : 3,
+               # QtCore.Qt.XButton1 : None,
+               # QtCore.Qt.XButton2 : None,
+               }
     def __init__( self, figure ):
         if DEBUG: print 'FigureCanvasQt: ', figure
         _create_qApp()
@@ -165,9 +170,10 @@ def mousePressEvent( self, event ):
         x = event.pos().x()
         # flipy so y=0 is bottom of canvas
         y = self.figure.bbox.height - event.pos().y()
-        button = self.buttond[event.button()]
-        FigureCanvasBase.button_press_event( self, x, y, button )
-        if DEBUG: print 'button pressed:', event.button()
+        button = self.buttond.get(event.button())
+        if button is not None: # only three buttons supported by MouseEvent
+            FigureCanvasBase.button_press_event( self, x, y, button )
+        if DEBUG: print('button pressed:', event.button())
 
     def mouseMoveEvent( self, event ):
         x = event.x()
@@ -180,9 +186,10 @@ def mouseReleaseEvent( self, event ):
         x = event.x()
         # flipy so y=0 is bottom of canvas
         y = self.figure.bbox.height - event.y()
-        button = self.buttond[event.button()]
-        FigureCanvasBase.button_release_event( self, x, y, button )
-        if DEBUG: print 'button released'
+        button = self.buttond.get(event.button())
+        if button is not None: # only three buttons supported by MouseEvent
+            FigureCanvasBase.button_release_event( self, x, y, button )
+        if DEBUG: print('button released')
 
     def wheelEvent( self, event ):
         x = event.x()