Expanding a macro in a custom action

[kumaichi]

I created a global macro:
<?xml version='1.0' encoding='utf-8'?>
<macros version='9'>
<macro name='TF_EXE_PATH'>
<description>Path, including the exe file to your local systems TF.exe</description>
<value>%DEVSTUDIO_NET_DIR%\Common7\IDE\TF.exe</value>
</macro>
</macros>

Inside my custom action (C#), I try to expand the macro like so:
IMacros macros = builder.App.get_Macros( MacroTypeEnum.vbldMacroGlobal );
builder.LogMessage( builder.App.ExpandMacros( Convert.ToString( macros[ "TF_EXE_PATH" ].Value ) ) );

When I run my step, I get the following error:
Exception: System.NotImplementedException: The method or operation is not implemented.
at System.Runtime.InteropServices.Marshal.ThrowExcept ionForHRInternal(Int32 errorCode, IntPtr errorInfo)
at System.Dynamic.ComRuntimeHelpers.CheckIfMissingTyp eInfoIsExpected(Int32 hresult, Boolean throwIfMissingExpectedTypeInfo)
at System.Dynamic.ComRuntimeHelpers.GetITypeInfoFromI Dispatch(IDispatch dispatch, Boolean throwIfMissingExpectedTypeInfo)
at System.Dynamic.IDispatchComObject.EnsureScanDefine dMethods()
at System.Dynamic.IDispatchComObject.System.Dynamic.I DynamicMetaObjectProvider.GetMetaObject(Expression parameter)
at System.Dynamic.DynamicMetaObject.Create(Object value, Expression expression)
at System.Dynamic.DynamicMetaObjectBinder.Bind(Object[] args, ReadOnlyCollection`1 parameters, LabelTarget returnLabel)
at System.Runtime.CompilerServices.CallSiteBinder.Bin dCore[T](CallSite`1 site, Object[] args)
at System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1)
at CallSite.Target(Closure , CallSite , Application , Object )
at System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1)
at VersionManager.Manager.CheckoutAIFile(String AssemblyInfoPath, IStep step, Builder builder)

I've tried using:
builder.ExpandMacros(...)
builder.App.ExpandMacros(...)
builder.App.ExpamdMacrosAndScripts(...)

All of them say that the method isn't implemented.

Any suggestions I could try?

Thanks,

Craig