I couldnt merge the custom sobject records records. Its shows error like 
Specified type Candidate__c cannot be merged

My Code is :
// Account creation code

List<Candidate__c> newCandidateList = new List<Candidate__c>();  
Candidate__c c1= new Candidate__c();  
c1.Years_of_experience__c = 4;  
c1.Last_Name__c = 'Oracle';  
c1.First_Name__c = 'Inc.';  
newCandidateList.add(c1);  
Candidate__c c2= new Candidate__c();  
c2.Years_of_experience__c = 14;  
c2.Last_Name__c = 'Oracle Inc';  
c2.First_Name__c = 'Inc.';  
newCandidateList.add(c2);  
Candidate__c c3= new Candidate__c();  
c3.Years_of_experience__c = 24;  
c3.Last_Name__c = 'Oracle Inc';  
c3.First_Name__c = 'India';  
newCandidateList.add(c3);  
insert newCandidateList;  
system.debug('Insert List are :' + newCandidateList);  

// Merging code  


   Candidate__c childrecords = [SELECT Id, Name, First_Name__c, Last_Name__c, Years_of_experience__c FROM Candidate__c WHERE First_Name__c = 'Inc.' LIMIT 1];  
system.debug('Retrieved Child Records are :' +childrecords);  
Candidate__c masterecord = [SELECT Id, Name, First_Name__c, Last_Name__c, Years_of_experience__c FROM Candidate__c WHERE First_Name__c = 'India' LIMIT 1];  
system.debug('Master Records :' + masterecord);  
try{  
	merge masterecord childrecords;      
} catch (Exception de){  
    system.debug('Error during merging the records :' + de.getMessage());  
}