Revit Convert Lines Greyed Out

I had something unusual happen to me today. I was working in a family and wanted to convert some model lines to symbolic lines. The problem was, the 'convert lines' button was greyed out, so Revit wouldn't allow me to convert the lines. (I couldn't get it to grey out again though, more below, but this is the button I am referring too).

After a bit of head scratching, I realised it was simply because Revit wasn't detecting a work plane to drop the line onto once converted. 

In writing this blog, I thought I would try and replicate the issue in a new family...except, I couldn't.
I tried removing reference plane names, deleting previous work planes and I even successfully got the Revit workplane to report <not associated>, yet I couldn't get the 'Convert Lines' button to grey out again. Weird. 

If I work out a way, or find something on the forums I will update the post, but for now it seems like one of those rare events. At least now I know what the fix is.