SQL & PL/SQL

Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 - 64bi
PL/SQL Release 10.2.0.5.0 - Production
CORE 10.2.0.5.0 Production
TNS for 64-bit Windows: Version 10.2.0.5.0 - Production
NLSRTL Version 10.2.0.5.0 - Production

Hi

I have a query like this that works fine and returns x amount of records;

select to_number(col_3) plot_number
  from user_recovery_external;

I now want a distinct list of values so I add distinct like this and it now throws an error;

select distinct to_number(col_3) plot_number
  from user_recovery_external;

Error starting at line 15 in command:
select distinct to_number(col_3) plot_number
  from user_recovery_external
Error report:
SQL Error: ORA-29913: error in executing ODCIEXTTABLEFETCH callout
ORA-01722: invalid number
ORA-06512: at "SYS.ORACLE_LOADER", line 52
29913. 00000 -  "error in executing %s callout"
*Cause:    The execution of the specified callout caused an error.
*Action:   Examine the error messages take appropriate action.

What am I doing wrong?

Ben