Dash core dump on network monitoring script

systemd-coredump[65616]: [🡕] Process 65608 (sb-nettraf) of user 1000 dumped core.

                                                 Stack trace of thread 65608:
                                                 #0  0x00007c545ec90278 n/a (libc.so.6 + 0x4a278)
                                                 #1  0x00005d3418d15df2 n/a (dash + 0x9df2)
                                                 #2  0x00005d3418d16057 n/a (dash + 0xa057)
                                                 #3  0x00005d3418d162e4 n/a (dash + 0xa2e4)
                                                 #4  0x00005d3418d16395 n/a (dash + 0xa395)
                                                 #5  0x00005d3418d16413 n/a (dash + 0xa413)
                                                 #6  0x00005d3418d15ecc n/a (dash + 0x9ecc)
                                                 #7  0x00005d3418d1673d n/a (dash + 0xa73d)
                                                 #8  0x00005d3418d20693 n/a (dash + 0x14693)
                                                 #9  0x00005d3418d1d5e0 n/a (dash + 0x115e0)
                                                 #10 0x00005d3418d13f57 n/a (dash + 0x7f57)
                                                 #11 0x00005d3418d13f57 n/a (dash + 0x7f57)
                                                 #12 0x00005d3418d14318 n/a (dash + 0x8318)
                                                 #13 0x00005d3418d13f57 n/a (dash + 0x7f57)
                                                 #14 0x00005d3418d13f57 n/a (dash + 0x7f57)
                                                 #15 0x00005d3418d13f1b n/a (dash + 0x7f1b)
                                                 #16 0x00005d3418d13f1b n/a (dash + 0x7f1b)
                                                 #17 0x00005d3418d13f1b n/a (dash + 0x7f1b)
                                                 #18 0x00005d3418d13f1b n/a (dash + 0x7f1b)
                                                 #19 0x00005d3418d14564 n/a (dash + 0x8564)
                                                 #20 0x00005d3418d1dae5 n/a (dash + 0x11ae5)
                                                 #21 0x00005d3418d13f57 n/a (dash + 0x7f57)
                                                 #22 0x00005d3418d16db9 n/a (dash + 0xadb9)
                                                 #23 0x00005d3418d20693 n/a (dash + 0x14693)
                                                 #24 0x00005d3418d1d5e0 n/a (dash + 0x115e0)
                                                 #25 0x00005d3418d13f57 n/a (dash + 0x7f57)
                                                 #26 0x00005d3418d1ca6a n/a (dash + 0x10a6a)
                                                 #27 0x00005d3418d10288 n/a (dash + 0x4288)
                                                 #28 0x00007c545ec6bcd0 n/a (libc.so.6 + 0x25cd0)
                                                 #29 0x00007c545ec6bd8a __libc_start_main (libc.so.6 + 0x25d8a)
                                                 #30 0x00005d3418d10435 n/a (dash + 0x4435)
                                                 ELF object binary architecture: AMD x86-64

The script is as follows:

#!/bin/sh

# Module showing network traffic. Shows how much data has been received (RX) or
# transmitted (TX) since the previous time this script ran. So if run every
# second, gives network traffic per second.

case $BLOCK_BUTTON in
	1) setsid -f "$TERMINAL" -e bmon ;;
	3) notify-send "🌐 Network traffic module" "🔻: Traffic received
🔺: Traffic transmitted" ;;
	6) "$TERMINAL" -e "$EDITOR" "$0" ;;
esac

update() {
    sum=0
    for arg; do
        read -r i < "$arg"
        sum=$(( sum + i ))
    done
    cache=/tmp/${1##*/}
    [ -f "$cache" ] && read -r old < "$cache" || old=0
    printf %d\\n "$sum" > "$cache"
    printf %d\\n $(( sum - old ))
}

rx=$(update /sys/class/net/[ew]*/statistics/rx_bytes)
tx=$(update /sys/class/net/[ew]*/statistics/tx_bytes)

# shellcheck disable=SC2046,2183,2086
printf "🔻%4sB 🔺%4sB\\n" $(numfmt --to=iec $rx $tx)