code: private Field checkFieldInAllSchemas(String name) throws TupleMRException {
      Field field = null;
      for (int i = 0; i < mrConfig.getIntermediateSchemas().size(); i++) {
        Field fieldInSource = checkFieldInSchema(name, i);
        if (field == null) {
          field = fieldInSource;
        } else if (field.getType() != fieldInSource.getType() || field.getObjectClass() != fieldInSource.getObjectClass()) {
          throw new TupleMRException("The type for field '" + name
              + "' is not the same in all the sources");
        } else if (fieldInSource.isNullable()) {
          // IMPORTANT CASE. Nullable fields must be returned when present nullable and non nullable fields mixed
          field = fieldInSource;
        }
      }
      return field;
    }
docs: Checks that the field with the given name is in all schemas, and
  select a representative field that will be used for serializing. In the case of
  having a mixture of fields, some of them nullable and some others no nullables,
  a nullable Field will be returned.