diff --git a/dateutil/rrule.py b/dateutil/rrule.py
index f9dcf62..2167d52 100644
--- a/dateutil/rrule.py
+++ b/dateutil/rrule.py
@@ -972,9 +972,9 @@
                                                ignoretz=ignoretz,
                                                tzinfos=tzinfos)
             except AttributeError:
-                raise "unknown parameter '%s'" % name
+                raise ValueError, "unknown parameter '%s'" % name
             except (KeyError, ValueError):
-                raise "invalid '%s': %s" % (name, value)
+                raise ValueError, "invalid '%s': %s" % (name, value)
         return rrule(dtstart=dtstart, cache=cache, **rrkwargs)
 
     def _parse_rfc(self, s,
diff --git a/dateutil/tz.py b/dateutil/tz.py
index 76ca4fc..5d5a089 100644
--- a/dateutil/tz.py
+++ b/dateutil/tz.py
@@ -714,9 +714,9 @@
         if tzid is None:
             keys = self._vtz.keys()
             if len(keys) == 0:
-                raise "no timezones defined"
+                raise ValueError, "no timezones defined"
             elif len(keys) > 1:
-                raise "more than one timezone available"
+                raise ValueError, "more than one timezone available"
             tzid = keys[0]
         return self._vtz.get(tzid)