255 return code from wait builtin

[Ian]

Hi,

When I wrap the wait builtin with a command substitution it returns a 255 code. 
Why is this?

# This seems normal
sleep 1 &
wait $!
echo $? # prints "0"

# This is surprising
sleep 1 &
a=$(wait $!)  # <---- only difference is cmd substitution
echo $? # prints "255"

Thanks