a2x --format manpage man/compton.1.asciidoc
a2x: ERROR: "xmllint" --nonet --noout --valid "/home/gregorystarr/Downloads/srcs/compton/man/compton.1.xml" returned non-zero exit status 4
make: *** [Makefile:146: man/compton.1] Error 1

This is the error I get whenever I try to build make docs on ubuntu 19.04. What is the problem?