/sbin/ifup has

        # set up default gateway
        if [ "${GATEWAY}" != "" -a "`ipcalc --network ${GATEWAY}
${NETMASK}`" = 
"NETWORK=${NETWORK}" ]; then
            route add default gw ${GATEWAY} ${DEVICE}
            DEFGW=${GATEWAY}
        elif [ "${GATEWAYDEV}" = "${DEVICE}" ]; then
            route add default ${DEVICE}
        fi

The problem is `ipcalc --network ${GATEWAY} ${NETMASK}` is always
evaluated with /bin/sh even if "${GATEWAY}" is "". I won't call it
a bash bug. One way to fix it is to use

`ipcalc --network ${GATEWAY} ${NETMASK} 2> /dev/null`

to get rid of error messages from ipcalc.