Finding classes tagged with a custom attribute doesn't work

I've used the code from another answer to find classes in an assembly that are tagged with a specific custom attribute:

var allClasses = tpAssy.GetTypes();

var testClasses = from t in tpAssy.GetTypes()
                  let attributes = t.GetCustomAttributes(typeof(TestClassAttribute), true)
                  where attributes != null && attributes.Length > 0
                  select new { Type = t, Attributes = attributes.Cast<TestClassAttribute>() };

However, in my test-case assembly I can see that allClasses lists 4 classes, of which three are tagged with the custom attribute (TestClassAttribute) that I am interested in. but the second Linq query assigned to testClasses returns an empty enumeration.

Here's the code header for one such class:

[TestClass()]
public class BaseSessionTest
{...

This is VS2013 with .Net 4.5.

What gives?