Comment 1 for bug 1215210

So the idea behind starting with empty window names is so that you can fit as many windows as possible horizontally across the tmux status line. Nulling those out enables you to fit many more windows.

You can always rename a window using the F8 keybinding, or tmux's ctrl-a-,

That said, I'll try to find a solution to your problem, where you're trying to rename the window yourself...