ORA-00932: inconsistent datatypes: expected - got CLOB

to satisfy union sql. I am using empty_clob function as below. but is showing error.

select * from

(select

    test_type test_type,

    result_type result_type,

    test_column param_name,

    result_column storage_col_name,

    '' value_data_type,

     empty_clob() as enum_data_type

from

    test_result_column_map

where

  mdf_id = 1029

)

union

(           

select

    test_type test_type,

    '' result_type,

    param_name param_name,

    param_column storage_col_name,

    value_data_type value_data_type,

    enum_data_type  enum_data_type

from

    test_param_column_map

where

      mdf_id = 1029

);