ExpectedException and AspNetHostingPermission attributes for testing Medium Trust ASP.Net environment

I'm trying to emulate a ASP.Net Medium Trust environment to make sure SecurityExceptions are handled properly. The first test passes ok because it catches the SecurityException:

[Test]
[ExpectedException(typeof(SecurityException))]
[AspNetHostingPermission(SecurityAction.PermitOnly, Level = AspNetHostingPermissionLevel.Medium)]
public void Can_only_access_certain_environment_variables_under_Medium_Trust()
{
    Environment.GetEnvironmentVariable("USERNAME");
}

This test throws a SecurityException even though I'm running under Full Trust:

[Test]
[AspNetHostingPermission(SecurityAction.PermitOnly, Level = AspNetHostingPermissionLevel.Unrestricted)]
public void All_environment_variables_are_accessible_under_Full_Trust()
{
    Environment.GetEnvironmentVariable("USERNAME");
}

Any ideas why the second test is failing?

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