NUnit Error: Not a test assembly

I have a solution with 3 projects in it (one is a C# class-library for unit tests, another is a MVC 2 web application)

I have included all the necessary references for NUnit, and I have correctly decorated the classes and methods with [TestFixture] and [Test] respectively.

It compiles with no errors.

I am using Visual Studio 2010 Pro

When I try to open UnitTests.dll in NUnit, I get the following error:

Not a test assembly. This assembly was not built with any known testing framework.