Classes cannot inherit

[Janaka]

I'm using Web Matrix and a custom built DLL which I've placed in my /bin
directory for my website on a Windows 2003 server. I'm using the same setup
which worked previously on Windows 2000. Any of the classes in the dll
which need to be instantiated work correctly.
for example the following is fine:

public class Tester
{
public int Add(int x, int y)
{return x + y;}
}

I also have a base class in my dll which i inherit from in all my aspx class
files. I've supplied a simplified base class for an example:

namespace MyNamespace.MCS
{
public class TestBase : Page
{
private string v;
public string Visitor
{
get{return v;}
}

public TestBase()
{}

protected void Page_Load(object sender, EventArgs e)
{
v = "12312klj31l2k3123";
}
}
}

Now my aspx files all have a source CS file and inherit from the CS (Note
to VS users this is not using CodeBehind). In the CS file I use the same
namespace, MyNamespace.MCS, and in the class definiation i set it to inherit
from TestBase - i.e. public class Default : TestBase

I keep getting the parser error Could not load type
'MyNamespace.MCS.Controls.Default'
Does anyone have any ideas as to why this might be happening?