Small gap between sash and pane

[blayet]

I am preparing to repaint the top surface of bottom part of a sash window (whatever that should be called). The bit where the condensation collects after dripping down from the pane... Having stripped off the paint on the interior side, there is a small gap maybe 2mm wide and 3 mm deep between the wood and the pane. Don't know whether that's localised decay or whether that's how it was originally. Anyway, just wondering what I should fill this with? Caulk, putty? Thanks for any advice
cheers
ben