Dear Virgil: stop trying to kill me when I bring you the stuff you need.

SPOILERS.

I've made it to the Institute and retrieved the experimental serum for Virgil. When I fast travel to his cave to deliver the serum, he and his defenses immediately go hostile and try to kill me.

I've tried this with power armor and a companion. Without power armor but with a companion. Without power armor and without any companion. No matter what, I get attacked.

I haven't joined the Institute, so I can't see how it's a faction alignment issue. I was already in with the Minutemen and the Brotherhood of Steel when I visited him the first time and he wasn't hostile. Being on good terms with the Railroad is new, but I can't think of a reason Virgil would be hostile to that faction.

Note: the forum threads I've seen seem to agree that going to Virgil with no power armor and no companion will allow you to deliver the serum. When I tried it, I still got attacked.

I know others have encountered this bug. Has anyone solved it by any other method than going to him sans power armor or companion?