Help: ALV Error "TABLE_INVALID_INDEX"

Hi

Me again, I have to change a customer id for customer name into an ALV list but when I change this

  read table e01_lt_fieldcat into ls_fieldcat                 
                               with key fieldname = 'CLIFI'   
                                        tabname   = 'T_POS'.  
  ls_fieldcat-seltext_l = 'ClienteFin'.                       
  sy_index = sy-tabix.                                        
  modify e01_lt_fieldcat index sy_index from ls_fieldcat.     

when I change CLIFI (customer ID) for NOMCLIFI (Customer name) the error TABLE_INVALID_INDEX appears. I will appreciate it any info, because it's an urgent specification.

Thanks in advance!!!

Tokio

P.D. SAP version is 4.0B