Adding simpletest prefix in settings fails

Adding simpletest prefix fails test with master slave configuration.

Settings.php code

$databases['fixture']['default'] = array(
'driver' => 'mysql',
'database' => 'simpletest',
'username' => 'root',
'password' => '',
'host' => 'localhost',
'port'=>'3306',
'simpletest_prefix' => 'simpletest999999',
);

Below is the error we are getting.

Argument 2 passed to SelectQuery::fields() must be an array, null given, called in /var/www/html/includes/entity.inc on line 284 and defined Recoverable error select.inc 1300 SelectQuery->fields() Exception
Argument 2 passed to SelectQuery::fields() must be an array, null given, called in /var/www/html/includes/entity.inc on line 284 and defined Recoverable error select.inc 1300 SelectQuery->fields() Exception
Argument 2 passed to SelectQuery::fields() must be an array, null given, called in /var/www/html/includes/entity.inc on line 284 and defined Recoverable error select.inc 1300 SelectQuery->fields() Exception
Argument 2 passed to SelectQuery::fields() must be an array, null given, called in /var/www/html/includes/entity.inc on line 284 and defined Recoverable error select.inc 1300 SelectQuery->fields() Exception
Undefined index: niojak Notice theme.inc 100 drupal_theme_initialize() Exception
Trying to get property of non-object Notice theme.inc 145 _drupal_theme_initialize()

Any thoughts on this???