Error code opening xls file

[Marco Marchi - Gmail]

Hello everybody,

I'm writing a script to process an Excel file.
During file opening with library function xls_open I obtain an error 
message:

  [fd, sst, sheetname, sheetpos] = 
xls_open('c:/documenti/sistemi/leggi/iso_sistemi/utilities/calls.xls');
                                                                                              !--error 999

XLS file I'm trying to open inside Scilab is 38 Mbyte: is it too much 
big for this function? I used xls_open in order to open smallest xls 
file with no problem.

What does 999 error code mean?

Thank you in advance,

Marco