I have following script to check free memory, However, I am getting ./memory_monitor.sh: line 6: [: 135Mi: integer expression expected error.

#!/bin/bash
THRESHOLD="500"
FREE_MEM=$(free -mh | awk '/^Mem:/{print $4}')

if [ "$FREE_MEM" -lt "$THRESHOLD" ]; then
        echo "insufficient storage. Available memory is ${FREE_MEM} MB"
fi