diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
index 3ef05b0aa12b..c6814e9f3019 100644
--- a/lib/matplotlib/backend_bases.py
+++ b/lib/matplotlib/backend_bases.py
@@ -1118,12 +1118,14 @@ def _timer_set_single_shot(self):
     def _on_timer(self):
         '''
         Runs all function that have been registered as callbacks. Functions
-        can return False if they should not be called any more. If there
+        can return False (or 0) if they should not be called any more. If there
         are no callbacks, the timer is automatically stopped.
         '''
         for func, args, kwargs in self.callbacks:
             ret = func(*args, **kwargs)
-            if not ret:
+            # docstring above explains why we use `if ret == False` here,
+            # instead of `if not ret`.
+            if ret == False:
                 self.callbacks.remove((func, args, kwargs))
 
         if len(self.callbacks) == 0: