How to constrain method to return type where : Attribute

I have an attribute

[System.AttributeUsage(System.AttributeTargets.Class]
public class ModuleValues : System.Attribute
{
    public Guid? Id { get; set; }

    public ModuleValues(string id)
    {
        Id = Guid.Parse(id);
    }
}

How can I create a generic method that returns the attribute once found?

public static T GetAttribute<T>(Type type) where T : Attribute
{
    object[] attrs = typeof(T).GetCustomAttributes(true);
    foreach (Attribute attr in attrs)
    {
        if (attr.GetType() == typeof (T))
        {
            //Console.WriteLine(attr.ToString());
            return attr as T;
        }
    }

    return default(T);
}

With the constraint in place, my method always returns null.

When I remove the constraint, I can find the attribute with a Console.WriteLine but cannot return it because the line return attr as T; gives compile error The type parameter 'T' cannot be used with the 'as' operator because it does not have a class type constraint nor a 'class' constraint

Example usage from my console:

Assembly x = Assembly.GetAssembly(typeof(MyBase));

Type[] types = x.GetTypes();
foreach (Type type in types)
{
    if (type.IsSubclassOf(typeof (MyBase)))
    {
        ModuleValues s = CustomAttributeHelper.GetAttribute<ModuleValues>(type);
        Console.WriteLine("S" + s);
    }
}