diff --git a/clients/discord.py b/clients/discord.py
index 21d6d9e..ed455f4 100644
--- a/clients/discord.py
+++ b/clients/discord.py
@@ -63,7 +63,7 @@ class DiscordClient(discord.Client):
             (re.compile(r'^!yt\s+(?P<q>")?(?P<query>.+)(?(q)")$'), self._do_yt),
             (re.compile(r'^!(find)?clip\s+(?P<q>")?(?P<filter>.+)(?(q)")$'), self._do_clip),
             (re.compile(r'^!cheese\s+(?P<q>")?(?P<query>.+)(?(q)")$'), self._do_cheese),
-            (re.compile(r'^!roll\s+(?P<q>")?(?P<formula>.+)(?(q)")$'), self._do_roll),
+            (re.compile(r'^!roll(\s+(?P<q>")?(?P<formula>.+)(?(q)"))?$'), self._do_roll),
             (re.compile(r'^!nextstream$'), self._do_nextstream),
         ]
         super(DiscordClient, self).__init__()
diff --git a/commands.py b/commands.py
index 3b37c5a..ee706b8 100644
--- a/commands.py
+++ b/commands.py
@@ -20,6 +20,7 @@ class Commands(object):
     def __init__(self, config, logger):
         self.config = config
         self.logger = logger
+        self.last_roll_formula = None
 
     def add_quote(self, id, text, game, date):
         try:
@@ -192,11 +193,14 @@ class Commands(object):
             return event
 
     def roll(self, formula):
+        if not formula:
+            formula = self.last_roll_formula
         try:
             result = Roll20.execute(formula)
         except Roll20Error:
             raise CommandError('failed to interpret or execute the formula')
         else:
+            self.last_roll_formula = formula
             return result
 
     def _get_instagram_media(self, params):
diff --git a/services/roll20.py b/services/roll20.py
index 8dce39f..f395762 100644
--- a/services/roll20.py
+++ b/services/roll20.py
@@ -676,5 +676,5 @@ class Roll20(object):
             except AttributeError:
                 calculated = num(result)
             return '{0} = __**{1}**__'.format(str(result), calculated)
-        except (parsy.ParseError, RuntimeError) as e:
+        except (parsy.ParseError, TypeError, RuntimeError) as e:
             raise Roll20Error(str(e))