Hi,

I have wrote a batch class and now i am writing test class for it but i am getting this error. Argument must be an object that implements Database.Batchable

Batch class

global class CBatchCalculateRatingsHistoricalData 
{
// Persistent variables
global Boolean hasErrors {get; private set;}

// Constructor
global CBatchCalculateRatingsHistoricalData()
{  
    this.hasErrors = false;
}

global Database.QueryLocator start(Database.BatchableContext BC)
{
  system.debug('Processing start method'); 
    // Query string for batch Apex
    String query = ''; 
    query += 'SELECT Id,name,Date_of_First_Order__c,Number_of_Orders__c FROM Account ';
    if(system.Test.isRunningTest())
        query += ' LIMIT 200';
    return Database.getQueryLocator(query);
}

global void execute(Database.BatchableContext BC, List<sObject> objectBatch)
{

}

global void finish(Database.BatchableContext BC)
{

}
}

test class

@isTest(seeAllData = true)
public class scheduledBatchable
{  
 static testMethod void Test_CBatchRatingsHistory()
  {
      System.Test.startTest();
     CBatchCalculateRatingsHistoricalData b = new CBatchCalculateRatingsHistoricalData();
     Database.executeBatch(b, 200);
     System.Test.stopTest();

 }

 }

please tell me what is the issue with this code. If i am wrong then please help me to get it right. 

Thanks

Anu