Cant compare strings in bash

I have written code:

empty=""
while read line
do
    if [ "$empty" == "$line" ] ; then
        echo "Empty line!!"
    fi
done

I run it using

sh checkEmptyLine.sh < input.txt

But I am getting the error as follows:

checkEmptyLine.sh: 4: [: unexpected operator

Please let me know where have I gone wrong

PS I have tried [[ ]], "${line}", and they are not working for me..

Thanks in advance :)