Notice: Undefined index: dependencies in MigrateGroup::getInstance()

Getting this when registering my own migrate classes...

Notice: Undefined index: dependencies in MigrateGroup::getInstance() (line 137 of .../migrate/includes/group.inc).

$group_name = 'MyMigrationGroup';
$group_title = 'My Group Migrations';
$class_name = 'MyMigration';
$machine_name = $class_name;

$arguments = array();
$arguments['group_name'] = $group_name;

...

MigrateGroup::register($group_name, $group_title);
Migration::registerMigration($class_name, $machine_name, $arguments);
$migration = Migration::getInstance($machine_name);

Hopefully this isn't from improper use of MigrateGroup::register or Migration::registerMigration. That might be resolved once #1965854: Update documentation for Migrate 2.6 is ready.