NUnit task attribute Fixture

In NUnit I can with Fixture attribute specify which namespaces that should be run. When I use TeamCity it says:

RunTests.msbuild Value from NUnit task attribute Fixture has been ignored because TeamCity NUnit task is used

MSBuild:
<UsingTask TaskName="NUnit" AssemblyFile="$(teamcity_dotnet_nunitlauncher_msbuild_task)" />
<NUnit Assemblies="AAA.IntegrationTests.dll" Fixture="$(FastFixture)" ToolPath="$(NUnitToolPath)" />

Is there a way round this? We are using TeamCity Professional Version 4.5.5 (build 9103)