The “Modern Family” writing staff went back to work this week on the 20th Century Fox lot but there’s a key family member missing: series co-creator/exec producer Christopher Lloyd.

Lloyd has not returned to work for the show’s sixth season as he has yet to reach an agreement on a new contract with producer 20th Century Fox TV. The specifics of the impasse are murky but it’s understood that Lloyd is looking for a shorter term than he has done in the past, possibly as short as one year. Reps for Lloyd and 20th TV declined comment.

Sources close to the situation said the sides are still in communication and both camps are hopeful that a deal will be hammered out. Lloyd’s departure or absence for an extended period would be a big loss for the ABC comedy as he is a well-loved figure in the “Modern Family” troupe and has been a pivotal player alongside co-creator and co-showrunner Steven Levitan. Levitan struck a new four-year overall deal with 20th TV earlier this year.

It’s no secret in the biz that there has been strain in recent seasons between Lloyd and Levitan, and that the two tend to divvy up the workload on episodes rather than working side by side as they once did. (Sources stressed that the hurdle at present is between Lloyd and the studio, not Levitan.)

As with any successful TV show, there have been other behind-the-scenes blowups. The six adult cast members filed a lawsuit against the studio in the summer of 2012 in a dramatic (and successful) bid to secure pay raises. But even that very public fight quickly became a distant memory once deals were worked out and production began anew on the show that has claimed four consecutive Emmy wins for best comedy series, among other accolades — not to mention syndication riches.

Despite the contract issue, Lloyd was present last week when 20th TV hosted a gathering to toast the show’s milestone-moment in its fifth season finale of featuring the wedding of gay couple Cameron and Mitchell, played by Eric Stonestreet and Jesse Tyler Ferguson.

As the “Modern Family” writers dig in to planning for season six, the hope is that this particular family squabble will be old news before spring turns to summer.