Downton Abbey creator and showrunner Julian Fellowes says that he’s unsure if the show will get a sixth season in 2015/2016.

Speaking in a new interview with The Wall Street Journal, Fellowes said that while Downton Abbey season 5 is certainly heading into production, season 6 is still up in the air.

The reason for the uncertainty? Fellowes has a new show at NBC called The Gilded Age, but he can’t begin working on it until Downton is finished.

“I haven’t written [Gilded] yet, but it’s about the old aristocracy, the Winthrops and the Stuyvesants and the new money of oil and gas and shipping in the 1870s. It will all be fiction – it won’t be real people – but when those families descended on New York, they took over.”

As for his thoughts on Downton Abbey heading into season 6: “I don’t know yet if there is a season six, but it’s not going to go on forever. It won’t be Perry Mason.”

Elsewhere in his interview with the WSJ, Fellowes addressed the departure of O’Brien (played by Siobhan Finneran) and the difficulty of keeping British actors around for long. “Here [in America] you can get actors for five or seven years. For some shows you have to sign a thing saying you are prepared for that when you go for an audition, when you haven’t even got the part! We just don’t have that. No agent will give you an actor for longer than three years in England.”

Downton Abbey season 4 began its run on PBS in the United States yesterday, and the show has already aired all eight episodes in the United Kingdom. Following the series 4 finale in the fall, ITV announced that the show had been commissioned for series 5. Filming should begin in the coming months for a fall 2014 debut in the U.K.