The declaration is

declare integer  BUICScan  in BUICAP32.DLL integer iJobType, ;
                             string lpFront, string lpLenFront, ;
                             string lpBack, string lpLenBack, ;
                             string lpCode, string lpLenCode

the calling code is:

    m.nretval=buicscan(THIS.nscan_opt,;
      THIS.cfront_fn, ;
      @temp_cfront_len, ;
      THIS.cback_fn, ;
      @temp_cback_len, ;
      @temp_cmicr_text, ;
      @temp_cmicr_len)

with the values translating to:
=buicscan(5,"F.TIF"," "," "," "," "," ")

It takes about 3 seconds and then bombs with the "dll caused exception"

Please tell me what the obvious thing I'm missing is.