sql - ORA-01722: Invalid number in oracle -

i have table called table_temp column amount varchar2(20) executing below query:

select to_number(amount) amt table_temp  dc in ('c','d'); 

result above query

amt ---- 0 123 511 485 0 

i want fetch records amt > o , using below query that

select amount amt table_temp  dc in ('c','d') , to_number(amount) > 0; 

getting error ora-01722: invalid number please suggest me..

if sure rows in table dc in 'c' , 'd' numeric try:

with temp_res    (select amount         table_temp        dc in ('c','d')) select amount amt    temp_res   to_number(amount) > 0;