Cannot return null for non-nullable field (EVEN THOUGH THIS FIELD IS NULLABLE)

Trying to do a basic query:
query Query {
threads {
id
createdAt
title
op
Posts
}
}

Here is the error:
"message": "Cannot return null for non-nullable field Thread.Posts.",

But here is this field:
@Field(() => [String])
Posts: [string];

What the hell is this bug?? Can you fix it?