Can't run UWP unit tests: Test adapter sent back a result for an unknown test case. Ignoring result

I'm running into an issue in running unit tests in a Universal Windows Platform unit test library. Specifically, I'm getting the following error message:

Test adapter sent back a result for an unknown test case. Ignoring result for [unit test method name]

Here are the things I've tried, but to no avail:

• Ensured the solution builds with the x86 platform; I've even tried x64 as the platform too, but the same error occurs.
• Ensured that the platform that the test runner is using matches the platform I build against (x86 or x64).
• Deleted the bin and obj folders.
• Restarted Visual Studio (numerous times).

Are there other things I can check for?

Thanks!