I have a solution with a bunch of projects, some of which also have test projects. These are all compiled for the `Any CPU` platform.

In TeamCity, I have a build configuration for NUnit 2.6.4, on which I specify `Platform: auto (MSIL)` and `Version: 4.0`.

When I run the build, the tests fail with the following message:

```
NUnit error running tests in 'C:\BuildAgent\work\a58fc6cd9bf63032\JsonApi.Tests\obj\Release\JsonApi.Tests.dll' assembly
Loading assembly is compiled for v4.0.30319, MSIL
NUnit runner runtime is v4.0.30319, x64

```

Even stranger, when I just now switched off "Run recently failed tests first" (and changed *nothing else*), I instead got the following:

```
NUnit error running tests in 'C:\BuildAgent\work\a58fc6cd9bf63032\JsonApi.Owin.Tests\obj\Release\JsonApi.Owin.Tests.dll' assembly
Loading assembly is compiled for v4.0.30319, MSIL
NUnit runner runtime is v4.0.30319, x86

```

Note that in one case, the runner is x86, while in the other it's x64. In neither case, do they successfully load an assembly compiled for MSIL.

I assume something is off with my build settings here, but what?