Well, it probably needs just a little liquid to avoid cavitation.
I just made Watermelon juice, when I cut up a full ("seedless", which typically is not seedless and contains the small white genetically-immatured seeds I imagine they are) watermelon right when I buy one, I only make pieces for eating of the easily cleaned "center cut" type section(s), and I just throw all the rest (seeds and melon, not outer rind) into the Blendtec to make juice. I do have to add like 6-8 onces to start it out, but then I reuse some of the juice with each carafe-full. At first I tried without water, but I probably needed a tamper for this use.
When I've made carrot juice, I add some water because I don't first cut into small pieces.... but I've never tried without water.