Jehovah's witnesses

A couple of Jehovah's witnesses just knocked on my door.

I asked, "Is it true you people don't believe in blood transfusions?"

One of them said, "That's correct sir."

I said, "That's a shame."

The other one said, "Why do you say that?"

I said, "Because

Spoiler:

if you bang on my door again at 8.30am on a Sunday morning you're both going to need one."