diff --git a/progressbar/__init__.py b/progressbar/__init__.py
index 7fffe69..100e41a 100644
--- a/progressbar/__init__.py
+++ b/progressbar/__init__.py
@@ -205,7 +205,7 @@ def _handle_resize(self, signum=None, frame=None):
 
     def percentage(self):
         'Returns the progress as a percentage.'
-        return self.currval * 100.0 / self.maxval
+        return self.currval * 100.0 / (self.maxval or 1)
 
     percent = property(percentage)