Spring Data REST getting exception when trying to save the data - spring

Question

I new to using Spring data rest , so even though this might be a very basic thing I would appreciate it if someone could point what is the mistake that i made or point me in the right direction .
I have an EmployeeRepository which contains the employee entities.
public interface EmployeeRepository extends CrudRepository<Employee,Long>{
#Query("select emp from Employee emp where emp.employeeId=?1")
Employee findById(String employeeId);
#Query("select emp from Employee emp where emp.officialEmailId = ?1")
Employee findByOfficialEmailId(String officialEmailId);
}
Below is a part of the employee entity delcaration .
public class Employee extends AbstractAuditingEntity{
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
#Version
private Long version;
#Size(min = 3, max = 30)
#NotBlank
#Column(unique=true,nullable=false)
private String employeeId;
#Size(min=3,max = 25)
#NotBlank
#Field(index=Index.YES, analyze=Analyze.NO, store=Store.YES)
private String lastName;
#Size(min=3,max = 25)
#Field(index=Index.YES, analyze=Analyze.NO, store=Store.YES)
#NotBlank
private String firstName;
#Size(min=3,max = 25)
#Field(index=Index.YES, analyze=Analyze.NO, store=Store.YES)
private String middleName;
}
In my service class I needed to get list of employee entities based on a condition and set a common property to all those .I tried it and it did not work so I tried something much simpler , I'm trying to set the middle name of all employees to a common value and save it to the db . Here is the code for it
#Transactional
public void tempFunction(){
Iterable<Employee> empList = employeeRepository.findAll();
for(Employee e : empList){
e.setMiddleName("newMiddleName");
employeeRepository.save(e);
}
}
I'm getting the below exception 2016-02-20 17:59:08.545 ERROR 533 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.transaction.TransactionSystemException: Could not commit JPA transaction; nested exception is javax.persistence.RollbackException: Error while committing the transaction] with root cause
java.lang.NullPointerException: null
And the stack trace shows the root cause to be a NullPointerException which I know is not present anywhere near where I'm invoking this code from either in my controller or service . Could someone point if there is any mistake in my code , thanks in advance .