Unit testing Error: could not add reference to the project containing the code to be test

Hello,

I try to create an unit testing. I use reference [using Microsoft.VisualStudio.QualityTools.UnitTestFramework;]. I got a message:

could not add reference to the project containing the code to be test. The reson: adding [promy project name] as a project to project reference would cause this project to reference itself, check that the unit test project is suitable for this kind of project

Do you have any ideas?