Instrumental Magnitude

One of the things that has puzzled me about one of the VPHOT functions is its reporting of instrumental magnitude of the target star. It always reports it as some negative number. For the current star I am observing, its instrumental magnitude is reported as - 6.667. Now if I take that number literally, it would imply that the VPHOT interprets the data from CCD camera on the SRO telescope of the AAVSOnet to mean that this star is about four orders of magnitude brighter than Venus. Which is obviously wrong....

So anyone have any idea how to interpret VPHOT's instrumental magnitude number correctly? I'd like to do some work with the photometric transforms for various filters on this scope, but I have no idea what (lower case) b, v, r, i, values I should be using in the various transform equations.