Hey George!
This error comes whenever you use any other serializable datatype for mapper key other than LongWritable. This happens only when using multiple mappers. Please have a look below for your reference.
![image image](https://attachment.freshdesk.com/inline/attachment?token=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6NDA2MDEyMzA3MCwiZG9tYWluIjoiZWR1cmVrYS5mcmVzaGRlc2suY29tIiwiYWNjb3VudF9pZCI6NDE0NTN9.zqGzNWa9BfOjrzYAxOMy3F9AmGa3wQhMIA2z1juPOOE)
![image image](https://attachment.freshdesk.com/inline/attachment?token=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6NDA2MDEyMzEwNywiZG9tYWluIjoiZWR1cmVrYS5mcmVzaGRlc2suY29tIiwiYWNjb3VudF9pZCI6NDE0NTN9.xSQZebZKMx1L9HCLggp0LV54ho_j5mLfkMQ4o2pCgDA)
![image image](https://attachment.freshdesk.com/inline/attachment?token=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6NDA2MDEyMzE3NywiZG9tYWluIjoiZWR1cmVrYS5mcmVzaGRlc2suY29tIiwiYWNjb3VudF9pZCI6NDE0NTN9.y2deh7XkR1gfOHbA2V0eGNH1L8aKWVZbMYfX1p5OLgg)
With the right datatypes, it should work fine. Make the changes shown in the screenshots above and execute the program.