If you get the following error when using devel’s dvm() in drupal 7 (or indeed, if you see it when not using dvm()):
Notice: Undefined variable: output in drupal_var_export()
Then try putting this line into you settings.php file:
ini_set('error_reporting', 'E_ALL ^ E_NOTICE');
It worked for us – thanks to citytree for the fix!