对于PersistFieldSql,GeneralizedNewtypeDeriving失败

我正在尝试定义Markdown newtype,并使用GeneralizedNewtypeDeriving自动定义新实例:

import Text.Markdown
import Yesod.Text.Markdown
import Database.Persist.Sql

newtype MarkdownNewT = MarkdownNewT { getMarkdown :: Markdown }
  deriving (Eq, IsString, Monoid, PersistField, PersistFieldSql)

对于PersistFieldSql以下消息,此操作失败:

Could not coerce from ‘m Markdown’ to ‘m MarkdownNewT’
  because ‘m Markdown’ and ‘m MarkdownNewT’ are different types.
  arising from the coercion of the method ‘sqlType’ from type
               ‘forall (m :: * -> *). Monad m => m Markdown -> SqlType’ to type
               ‘forall (m :: * -> *). Monad m => m MarkdownNewT -> SqlType’

这是由于GHC 7.8.2 的新角色功能吗？在那种特殊情况下,我不知道该怎么做,因为Markdown本身就是Text的新类型......

或者这与forallon有关sqlType？当所有其他实例成功自动派生时,出现此错误的原因是什么？

谢谢