weird mksh local array ?

outch.ksh

# i use MIRBSD KSH R54 2016/11/11

# i don't understand why if i uncomment this line
# set -u

# i get this message
# outch.ksh[21]: _m4a[@]: parameter not set
# as it seems _m4 works

_mma () {
    local _m4 _m4a _key _value
    _m4a=( "$@" )
    _m4=( )
    for _key in $import; do
        nameref _value=$_key
        _m4+=( "-D$_key=$_value" )
    done
    m4 "${_m4[@]}" "${_m4a[@]}"
}