Wasted nearly two hours trying to figure out why some simple bash script of mine wasn't working.

#!/bin/sh

HOSTNAME=$(hostname)
echo HOSTNAME=$HOSTNAME

if [ "$HOSTNAME" == "kiffins-laptop" ]; then echo "This is Kiffin's laptop"
elif [ "$HOSTNAME" == "georges-laptop" ]; then echo "This is George's laptop"
else
    echo 'Unknown hostname (exit)'
    exit 1
fi
echo "Success"
exit 0

Looks perfectly valid to me, so how come when I run the damned script I get the following error messages?

HOSTNAME=laptop
[: 11: laptop: unexpected operator
[: 11: laptop: unexpected operator
Unknown hostname (exit)

Need a hint?