diff --git a/Database/Persist/Postgresql.hs b/Database/Persist/Postgresql.hs
--- a/Database/Persist/Postgresql.hs
+++ b/Database/Persist/Postgresql.hs
@@ -178,8 +178,9 @@
                   LibPQ.TuplesOk -> return ()
                   _ -> do
                     msg <- LibPQ.resStatus status
+                    mmsg <- LibPQ.resultErrorMessage ret
                     fail $ "Postgresql.withStmt': bad result status " ++
-                           show status ++ " (" ++ show msg ++ ")"
+                           show status ++ " (" ++ (maybe (show msg) (show . (,) msg) mmsg) ++ ")"
 
                 -- Get number and type of columns
                 cols <- LibPQ.nfields ret
@@ -373,7 +374,7 @@
     getAltersU :: [(DBName, [DBName])]
                -> [(DBName, [DBName])]
                -> [AlterTable]
-    getAltersU [] old = map (DropConstraint . fst) old
+    getAltersU [] old = map DropConstraint $ filter (not . isManual) $ map fst old
     getAltersU ((name, cols):news) old =
         case lookup name old of
             Nothing -> AddUniqueConstraint name cols : getAltersU news old
@@ -384,6 +385,9 @@
                         else  DropConstraint name
                             : AddUniqueConstraint name cols
                             : getAltersU news old'
+
+    -- Don't drop constraints which were manually added.
+    isManual (DBName x) = "__manual_" `T.isPrefixOf` x
 
 getColumn :: (Text -> IO Statement)
           -> DBName -> [PersistValue]
diff --git a/persistent-postgresql.cabal b/persistent-postgresql.cabal
--- a/persistent-postgresql.cabal
+++ b/persistent-postgresql.cabal
@@ -1,5 +1,5 @@
 name:            persistent-postgresql
-version:         1.0.2
+version:         1.0.3
 license:         MIT
 license-file:    LICENSE
 author:          Felipe Lessa, Michael Snoyman <michael@snoyman.com>