MSTest: “File extension specified '.dll' is not a valid test extension”

I have a test project in VS2010, and I need to run the unit tests from command line. On one machine the following scenario works just fine:

1) Build the tests via "MSBuild test_project.csproj"

2) Run the tests via "MSTest /testcontainer:test_project.dll"

But on the other machine the 2nd step produced a weird error message, namely: "File extension specified '.dll' is not a valid test extension"

Any idea how to tackle this?

Thanks!