Add predecessor work item selection grayed out

I'm attempting to add a predecessor link from a selected work item in the "Roadmap" plan view mode. The selected work item is already a child of another work item. The predecessor work item is also a child. The 'Select Work Items' shows the matching work item, 628, but it is grayed out:

http://farm6.staticflickr.com/5032/6947320422_807452d2fa_z.jpg

What's interesting is that I can select the grayed out work item and can create the pedecessor link. The same behavior does not exist when selecting the predecessor link from the Links tab in the work item editor.