Conditional statement, "unary operator expected"

I am having a problem with an example script that is given in the textbook 'Unix and System Administration Handbook'. The script is below... I am receiving an error that states logscript: line 8: [: 1: unary operator expected.

# The log level is set in the global variable LOG_LEVEL. The choices
# are, from most to least severe.  Error, Warning, Info, and Debug.

function logMsg {
    message_level=$1
    message_itself=$2

    if [ $message_level -le $LOG_LEVEL ]; then
            case $message_level in
                    0) message_level_text="Error";;
                    1) message_level_text="Warning";;
                    2) message_level_text="Info";;
                    3)message_level_text="Debug";;
                    *) message_level_text="Other"
    esac
    echo "${message_level_text}: $message_itself"
    fi
}


# Main program starts here
logMsg $1 $2

Any ideas as to why this error is showing up?