Vista - MSBuild Project files and BuildUri

I have the following BuildStep in my msbuild proj file:

<Target Name="AfterCompile">
<ItemGroup>
<TestAssemblies Include="$(OutDir)\PBSMSDataStoreUnitTests.dll" />
</ItemGroup>

<BuildStep
Name="Run unit Tests"
TeamFoundationServerUrl="$(TeamFoundationServerUrl)"
BuildUri="$(BuildUri)"
Message="Running unit tests">
<Output TaskParameter="Id" PropertyName="StepId" />
</BuildStep>

<Gallio
Assemblies="@(TestAssemblies)"
ReportTypes="html"
ReportFileNameFormat="buildreport{0}{1}"
ReportOutputDirectory="." />

<BuildStep
TeamFoundationServerUrl="$(TeamFoundationServerUrl)"
BuildUri="$(BuildUri)"
Id="$(StepId)"
Message="Yay! All tests succeded!"
Status="Succeeded" />
<OnError ExecuteTargets="MarkBuildStepAsFailed" />
</Target>

<Target Name="MarkBuildStepAsFailed">
<BuildStep
TeamFoundationServerUrl="$(TeamFoundationServerUrl)"
BuildUri="$(BuildUri)"
Id="$(StepId)"
Message="Tests have failed. See test report in drop folder for details."

Status="Failed" />
</Target>

And when I run it in MSBuild I get the following error:

error MSB4044: The "BuildStep" task was not given a value for the required
parameter "BuildUri".

And I cannot see why, does anyone know?

Many thanks

Colin