VS Test step on TFS failing after all tests pass

I have a collection of MsTest and NUnit tests that are being run in TFS 2015 using the VS Test step. I'm using NUnit Test Adapter 3.4.1 to run the tests on the build agent.

At the end, even though the tests pass, Nunit seems to croak and the build step fails with these two errors.

2016-09-04T09:59:44.7209787Z ##[error]Error: Exception NUnit.Engine.NUnitEngineException, Exception thrown executing tests
2016-09-04T09:59:44.7209787Z ##[error]
2016-09-04T09:59:44.7209787Z ##[error]Error: Exception encountered unloading AppDomain
2016-09-04T09:59:44.7209787Z ##[error]
2016-09-04T09:59:44.7209787Z Information: NUnit Adapter 3.4.1.0: Test execution complete
2016-09-04T09:59:44.8615975Z Results File: C:\agent\_work2\1\TestResults\SRV-BLD1 2016-09-04 01_22_45.trx
2016-09-04T09:59:44.8615975Z Total tests: 139. Passed: 134. Failed: 0. Skipped: 5.

I've checked that there are indeed 139 tests in the suite, and 5 are set to ignore (2 are MSTest, and 3 are NUnit).

I'm not sure if there's a place to get more detailed explanation of the error. Searching this site and google seems to suggest that the NUnit.Engine.NUnitEngineException is linked to test discovery (here, and here for instance), but my tests are being discovered, so I'm not sure if this is related (Pretty new to NUnit, so not sure of a lot of things).

I also saw two links dealing with Adapter failures (here and here), but the errors don't quite match up, although maybe just because I don't have the same level of verbosity.

In TFS, the step doesn't have any configuration on it, just that it's enabled and the path to the DLLs.

Does anyone know what is causing the errors (which I'm assuming are causing the build to fail)? Alternatively, what should be the next steps in getting a more precise/verbose error stack to investigate the issue?

As a side note, I saw this SO answer, which states this:

MSTest.exe returned an exit code of 1 indicating that not all tests passed.

I wasn't able to find any confirmation that VSTests fails when it encounters Skipped tests, but could this also be an issue?

Thank you for any help.