Neomutt goes to next message on page down, but not previous on page up

Neomutt seems to have a strange feature: When I open a message with space, I can press PageDown to scroll down by a page. However, when this gets to the end of the message, it doesn't stop - it skips to the next message. But PageUp never goes to the previous message, and instead stops at the top. Is there a way to make PageDown **not** go to the next message?