PostProcess Build script fails when running from command line

I am trying to build my project from the command line targeting iOS.
When I build this from the editor, everything works! I have a PostProcess build script that I use.

When I try to build form the command line using the following command:
/Applications/Unity/Unity.app/Contents/MacOS/Unity -quit -batchmode -executeMethod BuildScript.BuildIOS -nographics -buildTarget iOS

I am getting an error from my PostProcess build script saying:
Assets/Editor/MyBuildPostprocessor.cs(9,23): error CS0234: The type or namespace name `Xcode' does not exist in the namespace `UnityEditor.iOS'. Are you missing an assembly reference?

The line that generates the error is a simple using clause:
using UnityEditor.iOS.Xcode;

So it seems that for some reason, the command line build mechanism does not recognise the UnityEditor.iOS.Xcode assembly, and my PostProcess build script cannot run.

Any ideas?

 

Am I the only one with this problem?!