Target does not exist when building .sln file, but does with .csproj

I created a target called RunNUnitTests and placed it at the bottom of  Micirosoft.CSharp.targets.

  <Target Name="RunNUnitTests">
     <ItemGroup>
        <TestAssembly Include="$(TEST_ASSEMBLY)"/>
     </ItemGroup>
     <NUnit Assemblies="@(TestAssembly)" HaltIfTestFailed="True" Platform="x86"/>
  </Target>

The build configuration defines a environment variable called TEST_ASSEMBLY  i.e.  path\path\path\MyTests.dll

I setup a build configuration referencing the .sln file of my tests with the Targets "Build RunNUnitTests".
The solution fails with this message :

• C:\BuildAgent\work\581b6fc1e96d4c06\Build\Greetings\GreetingsImpl\GreetingsImplTester.sln error MSB4057: The target "RunNUnitTests" does not exist in the project.

If I build with the .csproj file, then it finds the target just fine.

I even tried moving the target into the csproj file.  The solution still fails.

This seems similar to this thread :
http://www.jetbrains.net/devnet/thread/273625

Is this a bug or is there are fix for this?

Thanks,

Scott Knowles