How insert a varchar ' / / ' as null on date column in PostgreSQL 9 -

I have a temporary table and want to insert data from another table, but temp In the date type, and column in usomultiple is a varchar .

If I try to convert using the to_date () function, I get an error because thousands of cells have been found to be '/ /' Was empty as the value.

ERROR: Price is not valid for an «/ A» «DDA»

  temporarily from INSERT (cd_id, ds_estado, Dt_date) Select um.cd_uso, um.ds_estado, to_date (um.dt_dateIn, 'dd / MM / yyyy') from our liberalities;   
 How can I set the cell to poblated when the  '/ /'  and the correct value is set instead?  
 
 
 
  TO_DATE (NULLIF (dt_dateIn, '//'), 'dd / MM / yyyy ')