--- a/rollbot.py	Sun May 22 21:48:18 2016 +0800
+++ b/rollbot.py	Fri Jun 03 11:23:45 2016 +0800
@@ -39,8 +39,9 @@
                 if times is None:
                     times = 1
                 sides = match.group('sides')
-                rolls, total = self.roll(int(times), int(sides))
-                return self.format(rolls, total)
+                if times > 0 and sides > 1:
+                    rolls, total = self.roll(int(times), int(sides))
+                    return self.format(rolls, total)
 
         return '%s usage: %s' % (self.name, self.usage())