Nathan Fillion thanks fans following the "Castle" series finale

Nathan Fillion has always been sure to show his appreciation for his fans.

Fillion's crime drama, "Castle," signed off for the last time Monday night, as the series ended its eighth season just days after it was announced it would not be returning for a ninth. And Fillion was there on Twitter to live-tweet the entire thing.

He ended the night with a special note to his fans, thanking them for their support. "Well. That's it. Thank you, everyone. What an incredible journey. "(Mic drop)" Castle out," he tweeted.

But some fans, it seemed were more interested in an apology than a thank you, especially since several took issue with Fillion tweeting about events happening on the show before they had a chance to watch it. The ever-playful actor, though, kept the mood light.

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"But what are they going to do? Fire me?" Fillion tweeted after one fan complained.

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The folks behind "Castle" were prepared for the worst, as they reportedly shot two versions of the finale -- one for if the show was renewed for another year and one for if it wasn't. But even if the show had continued, co-star Stana Katic would not have been around next season, as her exit was announced last month.