man causes a broken pipe when using a pager other than less

Question

I'm trying to write a simple script to format my man pages, but I've run into a broken pipe error.

These lines both cause the error:

env MANPAGER=cat man $foo
man -P cat $foo

and this is the script I'm ultimately trying to use (converts underlines into italics)

man -P ul "$1" | sed -e $'s/\033\\[4m/\033\\[3m/g' -e $'s/\033\\[24m/\033\\[23m/g'

I get the same problem with cat, ul, and tee, and the only thing that seems to work is less.