head: head /proc/self/men file does not show content of file

[oech3]

$ gnu-9a3479f1e/head /proc/self/mem a
==> /proc/self/mem <==
head: error reading '/proc/self/mem': Input/output error

==> a <==
a

$ uutils-head /proc/self/mem a
==> /proc/self/mem <==
head: error writing 'standard output': Input/output error (os error 5)