Microsoft.VisualStudio.Text.UI.Wpf reference not recognized

I'm trying to implement IWpfTextViewCreationListener using a newly installed VS SDK 2013 running on VS2013 update 4.

All my references are resolving, except I get the error:
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Error3 The type 'Microsoft.VisualStudio.Text.Editor.IWpfTextViewCreationListener' is defined in an assembly that is not referenced. You must add a reference to assembly 'Microsoft.VisualStudio.Text.UI.Wpf, Version=12.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.

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I *do* have the offending DLL in my references, along side the rest of the VS SDK assemblies that are resolving fine. 

This must be something simple...