I keep getting gunned down by an invisible sentry

In TF2 (in the 2014 Halloween map, where I have to deliver the tickets, for what it's worth), I keep getting destroyed by a mini-sentry, but when I die, instead of zooming in on the sentry that killed me, the camera zooms in on what appears to be an invisible sentry. I've tried launching rockets at this thing, but it was to no avail. Is there any way to destroy these invisible sentries?