B737-800 First Officer reported a cabin door warning light during the climb on a cargo flight with no cabin crew or passengers. The flight returned to the departure airport and landed.

Narrative

While climbing out of ZZZ heading to ZZZ1; we was passing though an altitude of 9;800 ft when we got the AFT Entry light illumination. After that I advised the flying pilot to level off since we have an indication of a door not possibly be securely being closed. I then advised ATC that most likely have to return to ZZZ because we have a door open indication. ATC asked if we needed to declare and emergency at which time we advised him that we did not need to declare an emergency. ATC then advised us to descend and maintain 3000. We then started to descend to 3000. I then told my first officer that he had the radios and controls and I was going to reference the QRH. ATC asked us to advise when we was ready to go back to the airport. After reading then QRH I told we let ATC know that we was ready to head back to the airport. He started giving us vectors as we set the flight deck up for landing. Did a decent and approach check list then the before landing check list. Once we had the airport in sight we was cleared for the visual approach and to contact tower. Tower then cleared us to land on RWY XXL. After landing we taxied back to the ramp where maintenance eventually came out to check the door. During flight the QRH gave an option to go back and check the door handle or if unable to do so; then land at the nearest suitable airport. We had freight loaded in the back so we was unable to go back and check so the QRH advised to land at the nearest suitable airport so we returned to ZZZ.