Why does sysopen not assign the fd param in this case?

Why doesn't sysopen assign the fd parameter during startup, yet it
does assign it correctly on the command line afterwards?

% cd $(mktemp -d)
% > .zshrc <<EOF
zmodload -F zsh/system b:sysopen
PS1='%# '
PS4=+
set -x
sysopen -r -o cloexec -u fd <( sleep 1 )
print fd=$fd
EOF
% env -i HOME=$PWD PATH=$PATH TERM=$TERM ${TERMINFO:+TERMINFO=$TERMINFO} zsh -d
+sysopen -r -o cloexec -u fd /dev/fd/12
+print 'fd='
fd=
+sleep 1
% sysopen -r -o cloexec -u fd <( sleep 1 ); print fd=$fd
+sysopen -r -o cloexec -u fd /dev/fd/11
+print 'fd=15'
fd=15
% +sleep 1