diff --git a/app/models.py b/app/models.py
index fbd1b7419..cf58a6867 100644
--- a/app/models.py
+++ b/app/models.py
@@ -1191,7 +1191,13 @@ class Notification(db.Model):
     reply_to_text = db.Column(db.String, nullable=True)
 
     postage = db.Column(db.String, nullable=True)
-    CheckConstraint("notification_type != 'letter' or postage in ('first', 'second')")
+    CheckConstraint("""
+        CASE WHEN notification_type = 'letter' THEN
+            postage in ('first', 'second')
+        ELSE
+            postage is null
+        END
+    """)
 
     __table_args__ = (
         db.ForeignKeyConstraint(
@@ -1445,7 +1451,13 @@ class NotificationHistory(db.Model, HistoryModel):
     created_by_id = db.Column(UUID(as_uuid=True), db.ForeignKey('users.id'), nullable=True)
 
     postage = db.Column(db.String, nullable=True)
-    CheckConstraint("notification_type != 'letter' or postage in ('first', 'second')")
+    CheckConstraint("""
+        CASE WHEN notification_type = 'letter' THEN
+            postage in ('first', 'second')
+        ELSE
+            postage is null
+        END
+    """)
 
     __table_args__ = (
         db.ForeignKeyConstraint(
diff --git a/migrations/versions/0230_noti_postage_constraint.py b/migrations/versions/0230_noti_postage_constraint.py
new file mode 100644
index 000000000..118d51b39
--- /dev/null
+++ b/migrations/versions/0230_noti_postage_constraint.py
@@ -0,0 +1,44 @@
+"""
+
+Revision ID: 0230_noti_postage_constraint
+Revises: 0229_new_letter_rates
+Create Date: 2018-09-19 11:42:52.229430
+
+"""
+from alembic import op
+
+
+revision = '0230_noti_postage_constraint'
+down_revision = '0229_new_letter_rates'
+
+
+def upgrade():
+    op.execute("""
+        ALTER TABLE notifications ADD CONSTRAINT "chk_notifications_postage_null"
+        CHECK (
+            CASE WHEN notification_type = 'letter' THEN
+                postage in ('first', 'second')
+            ELSE
+                postage is null
+            END
+        )
+        NOT VALID
+    """)
+    op.execute("""
+        ALTER TABLE notification_history ADD CONSTRAINT "chk_notification_history_postage_null"
+        CHECK (
+            CASE WHEN notification_type = 'letter' THEN
+                postage in ('first', 'second')
+            ELSE
+                postage is null
+            END
+        )
+        NOT VALID
+    """)
+    op.execute('ALTER TABLE notifications VALIDATE CONSTRAINT "chk_notifications_postage_null"')
+    op.execute('ALTER TABLE notification_history VALIDATE CONSTRAINT "chk_notification_history_postage_null"')
+
+
+def downgrade():
+    op.drop_constraint('chk_notifications_postage_null', 'notifications', type_='check')
+    op.drop_constraint('chk_notification_history_postage_null', 'notification_history', type_='check')