If you pass an empty adapterNamespace value to Zend_Db::factory() it will throw an exception because it throws out the default prefix.

i.e line 229 of Db.php reads

      $adapterNamespace = 'Zend_Db_Adapter';
      if (isset($config['adapterNamespace'])) {
          $adapterNamespace = $config['adapterNamespace'];
          unset($config['adapterNamespace']);
      }

but should probably read

      $adapterNamespace = 'Zend_Db_Adapter';
      if (isset($config['adapterNamespace']) &&

($config['adapterNamespace']!='')) { $adapterNamespace = $config['adapterNamespace']; unset($config['adapterNamespace']); }

I created this problem by reading in and using the following xml config

<?xml version="1.0"?> My App (Dev Mode)Pdo_MysqllocalhostZWareZWareZWareMy test AppMy live App

Removing the tags from the config file resolves the issue. Why would I want empty tags in the config? Because I am setting the pattern for the config file. It may also be the case that other parameters need to be similarly checked.