System.Web.Providers doesn't work in medium trust

I have an ASP.NET WebForms app using Microsoft ASP.NET Universal Providers (NuGet) for membership and roles. Under full trust everything works fine, but when I edit web.config to specify medium trust I get this error when loading a page that accesses the membership:

Type 'System.Web.Providers.DefaultMembershipProvider' cannot be instantiated under a partially trusted security policy (AllowPartiallyTrustedCallersAttribute is not present on the target assembly).

I've searched the web and haven't found much info about whether this is a known limitation or if there is a workaround. I'm working on the open source app Gallery Server Pro, which is distributed in the MS Web Gallery and must support medium trust environments.

Any insight?

[Edit] Per the request, here is the web.config stuff (I'm not using profiles or the session state provider):

<membership defaultProvider="DefaultMembershipProvider">
  <providers>
    <clear />
    <add name="DefaultMembershipProvider" applicationName="Gallery Server Pro" connectionStringName="GalleryDb" passwordFormat="Clear" enablePasswordRetrieval="true" enablePasswordReset="true" requiresQuestionAndAnswer="false" requiresUniqueEmail="false" maxInvalidPasswordAttempts="50" minRequiredPasswordLength="2" minRequiredNonalphanumericCharacters="0" passwordAttemptWindow="10" type="System.Web.Providers.DefaultMembershipProvider, System.Web.Providers, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
  </providers>
</membership>
<roleManager enabled="true" cacheRolesInCookie="true" cookieProtection="Validation" defaultProvider="DefaultRoleProvider">
  <providers>
    <clear />
    <add name="DefaultRoleProvider" applicationName="Gallery Server Pro" connectionStringName="GalleryDb" type="System.Web.Providers.DefaultRoleProvider, System.Web.Providers, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364