drush sql-dump gives warning When I ran drush sql-dump --result-file, it came back with a warning and a success message.

system(): Cannot execute a blank command exec.inc:36                 [warning]
Database dump saved to                                               [success]
/my/homedir/drush-backups/unknown/20150113014628/_20150113_014628.sql                      


However, I checked inside the directory and no .sql backup file can be found. I have also read this issue, but I am not using drush aliases.

How can I fix this and backup my database using the command line?
I was following the docs, which told me to execute the command in my 'From your Drupal directory', so I executed the command on my Drupal installation root directory.

I have a multi-site set up, and running it inside my site directory /sites/mysite/ worked.