Online Trolls Want 'Castle' Star's Husband Dead

"Castle" star Stana Katic and her husband are getting harassed so viciously they've had to call in the LAPD.

Law enforcement sources tell us Katic's husband, Kris, went to police to report an IG user who'd been leaving nasty comments on Stana's social media. We're told some of the comments read, "Kris, I hope you die" and "I wish cancer on you and your family."

Our source says the harassment started in October, but things got really bad in February when the suspect started making nasty comments about extended family members and their dog by name. Stana and Kris are worried because those names aren't listed publicly.