diff --git a/persistable-record.cabal b/persistable-record.cabal
--- a/persistable-record.cabal
+++ b/persistable-record.cabal
@@ -1,5 +1,5 @@
 name:                persistable-record
-version:             0.5.0.2
+version:             0.5.1.0
 synopsis:            Binding between SQL database values and haskell records.
 description:         This package contiains types to represent table constraints and
                      interfaces to bind between SQL database values and Haskell records.
diff --git a/src/Database/Record/ToSql.hs b/src/Database/Record/ToSql.hs
--- a/src/Database/Record/ToSql.hs
+++ b/src/Database/Record/ToSql.hs
@@ -153,7 +153,7 @@
 @
 
 -}
-class ToSql q a where
+class PersistableWidth a => ToSql q a where
   -- | Derived 'RecordToSql' printer function object.
   recordToSql :: RecordToSql q a
 
@@ -178,7 +178,7 @@
 
 -- | Implicit derivation rule of 'RecordToSql' printer function object which can convert
 --   from Haskell 'Maybe' type into list of database value type ['q'].
-instance (PersistableType q, PersistableWidth a, ToSql q a) => ToSql q (Maybe a)  where
+instance (PersistableType q, ToSql q a) => ToSql q (Maybe a)  where
   recordToSql = maybeRecord persistableType persistableWidth recordToSql
 
 -- | Implicit derivation rule of 'RecordToSql' printer function object which can convert