diff --git a/core/admin/mailu/models.py b/core/admin/mailu/models.py
index 9b308b32..f9fabb7f 100644
--- a/core/admin/mailu/models.py
+++ b/core/admin/mailu/models.py
@@ -75,7 +75,7 @@ class CommaSeparatedList(db.TypeDecorator):
     """ Stores a list as a comma-separated string, compatible with Postfix.
     """
 
-    impl = db.Text
+    impl = db.String(255)
     cache_ok = True
     python_type = list
 
@@ -96,7 +96,7 @@ class JSONEncoded(db.TypeDecorator):
     """ Represents an immutable structure as a json-encoded string.
     """
 
-    impl = db.Text
+    impl = db.String(255)
     cache_ok = True
     python_type = str