Appending a line with slashes to a file using sed - bash

Question

I want to insert text before the first line of a file. I want to use sed and command sed -i '1s/^/line_to_be_added\n/' file.txt works fine, but when I want to add directory path and use slashes in command I get an error sed: -e expression #1, char 9: unknown option tos'. My command sed -i '1s//var/log/firstlogs/logs/first.json\n/' file.txt How to escape slashes in this case?