Cessna Citation Latitude Captain reported uncommanded movement of the aircraft after parking. Captain set the parking brake.

Narrative

We pulled into the FBO at ZZZ; and were parked by a line technician at the runway side of the ramp on a moderate up slope. After shutting down the engines; I called for the Shutdown with APU" checklist. SIC called the first response 'Parking Brake' and I looked down at the CAS (Crew Alerting System) messages and noticed the parking brake wasn't set. I immediately pressed the brake pedals and the aircraft came to a stop with a jerk. We set the parking brake and completed the checklist. Our passenger was not phased at all but our line technician said we had rolled backwards approximately three feet. Fortunately no damage was done and no personnel were injured. The line technician did not need to move us forward in our parking spot. Suggestions: Fatigue was part of the problem here; we haven't been getting much rest; short turnarounds have been combined with multiple time zones. The misstep could have been much worse. Fortunately the checklist as a control filled a hole in the swiss cheese today. In the future when I stop the aircraft I will verify the PARKING BRAKE CAS is illuminated before I release the brake pedals."