Update employee using custom function

Update employee using custom function

I'm trying to update employee records using a custom function in Zoho People. Here is my function:
  2. updateMap = Map();
  3. updateMap.put("EmployeeID", "XXXXXX");
  4. updateMap.put("First_Name", "KKKK");
  5. info updateMap;
  6. response = zoho.people.update("employee", updateMap);
  7. info response;

When I try to run this function I always get this error:

{"response":"{\"message\":\"Error occurred\",\"uri\":\"/api/forms/json/employee/updateRecord\",\"errors\":{\"code\":7204,\"message\":\"Wrong datatype for the Parameter Input\"},\"status\":1}"}
 
I'm just trying to update a simple field whose data type is String. What is the meaning of this error? I tried to update different fields and always get the same error.