In the web.config file did you set the defaultprovider in the membership configuration?

<membership defaultProvider="CustomizedProvider">
<providers>
<clear/>
<add connectionStringName="MyConnectionString" applicationName="MyApp" 
requiresQuestionAndAnswer="false" minRequiredPasswordLength="5" 
minRequiredNonalphanumericCharacters="0" name="CustomizedProvider" 
type="System.Web.Security.SqlMembershipProvider"/>

</providers>
</membership>