asp.net-membership - SqlMembershipProvider.GetPasswordWithFormat のエラー

Question

asp.net ログイン ページに次のエラーがあります。

Unable to cast object of type 'System.Int32' to type 'System.String'

スタック トレースあり:

at System.Data.SqlClient.SqlBuffer.get_String()     
   at System.Web.Security.SqlMembershipProvider.GetPasswordWithFormat(String username, Boolean updateLastLoginActivityDate, Int32& status, String& password, Int32& passwordFormat, String& passwordSalt, Int32& failedPasswordAttemptCount, Int32& failedPasswordAnswerAttemptCount, Boolean& isApproved, DateTime& lastLoginDate, DateTime& lastActivityDate)     
   at System.Web.Security.SqlMembershipProvider.CheckPassword(String username, String password, Boolean updateLastLoginActivityDate, Boolean failIfNotApproved, String& salt, Int32& passwordFormat)     
   at System.Web.Security.SqlMembershipProvider.ValidateUser(String username, String password)     
   at System.Web.UI.WebControls.Login.AuthenticateUsingMembershipProvider(AuthenticateEventArgs e)     
   at System.Web.UI.WebControls.Login.AttemptLogin()     
   at System.Web.UI.WebControls.Login.OnBubbleEvent(Object source, EventArgs e)     
   at System.Web.UI.Control.RaiseBubbleEvent(Object source, EventArgs args)     
   at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)     
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

毎回エラーになるわけではありません。

何か案は？