The drush command 'sql dump' could not be found Im a trying to do a backup of my drupal site with:

drush -v sql dump --result-file=DrupalBackups/nameofsqldb5.sql


but it results in following error message:

The drush command 'sql dump' could not be found.  Run `drush cache-clear drush` to 
[error] clear the commandfile cache if you have installed new extensions.


I ran drush cache-clear drush but without success.

Any ideas?
The actual command is sql-dump not sql dump

drush -v sql-dump --result-file=DrupalBackups/nameofsqldb5.sql


From drush help:


  sql-dump              Exports the Drupal DB as SQL using mysqldump or
  equivalent.