Castle stars Nathan Fillion and Stana Katic are known for their chemistry onscreen, though a new source is claiming they might not be so chummy behind the scenes.

Rick Confronts 'Old Wounds' In 'Castle' Season 7 Finale

Fillion and Katic, who play Castle and Beckett, respectively, on the ABC crime procedural apparently don't have the best relationship, according to one alleged source. The source spoke with the Daily Mail and said there is a lot of tension between them.

"The Castle cast just found out the show will be picked up for another season," the source alleged. "It's no secret that there is no love lost between these two, Things got so bad between them this past season that when one of them walked onto the set, the other one walked off."

Will 'Castle' Get Another Season? Jon Huertas Thinks So

The source added, "In fact, tensions became so high that Stana, who plays Kate Beckett, just didn't want to deal with the stress anymore, so when Nathan agreed to sign on for two more seasons, she made the decision not to sign again at all."

Fillion just signed the deal to extend his contract past season 7, while Katic has yet to officially sign the dotted line. Could it be that her strained relationship with Fillion is what's causing her hesitation?

"No one is sure just how this will play out," the source said. "There is always a possibility Stana will make a couple guest appearances during the season, but as far as her being back in her weekly role - it's just not going to happen."

It is unknown how true these claims really are. It's possible that this is all just idle gossip strummed up in the midst of contract negotiations. However, if it's true, ABC may choose to not go forward into season 8, especially since Castle and Beckett's relationship is so integral to the show.

Castle airs Mondays at 10 p.m. ET/PT on ABC.

Watch the promo for the next episode of Castle here: