oracle: ora-00932 converting column_expression user_ind_expressions controlling to_lob

try controlling twin rudimentary statements seer 10.2:

create list mytest
   (table_name varchar2(30), 
   index_name varchar2(30), 
   column_expression clob,
   column_position number);

insert mytest 
  (table_name,
   index_name,
   column_expression, 
   column_position) 
select table_name, index_name,
       to_lob(column_expression), column_position 
from user_ind_expressions expra 
where exists 
   (select 1 user_constraints 
    where constraint_name = expra.index_name 
    table_name = expra.table_name);

this law error:

error during line 1:
ora-00932: unsuitable datatypes: approaching - got long

if i replace where exists this:

insert mytest 
   (table_name,index_name,column_expression, column_position) 
select table_name,index_name,
       to_lob(column_expression), column_position 
from user_ind_expressions expra;

it works:

23 rows created.

what going on?