diff --git a/src/conf.py b/src/conf.py
index 74bacb144..3239af684 100644
--- a/src/conf.py
+++ b/src/conf.py
@@ -423,6 +423,7 @@ supybot.drivers.register('poll', registry.Float(1.0, """Determines the default
 length of time a driver should block waiting for input."""))
 
 class ValidDriverModule(registry.OnlySomeStrings):
+    validStrings = ('socketDrivers', 'twistedDrivers', 'asyncoreDrivers')
     def normalize(self, s):
         # We can't be case insensitive here.  At least not without work.
         return s 
diff --git a/src/registry.py b/src/registry.py
index 95abb13eb..06b18e0b5 100644
--- a/src/registry.py
+++ b/src/registry.py
@@ -299,6 +299,11 @@ class String(Value):
 class OnlySomeStrings(String):
     normalize = staticmethod(str.lower)
     validStrings = ()
+    def __init__(self, *args, **kwargs):
+        assert self.validStrings, 'There must be some valid strings.  ' \
+                                  'This is a bug.'
+        String.__init__(self, *args, **kwargs)
+        
     def setValue(self, s):
         s = self.normalize(s)
         if s in self.validStrings: