Problem with ENVI_CONVERT_File_COORDINATES

Hi,
I am having trouble to retrive a pixel value by geographic long lats.
Following is the code. two values obtained by sample/line coordinate
and geographic long lats are different. What is wrong with my code?
Thanks

 ;======================================================
  ;Open a file using envi batch

 envi_open_file, fNameInput, r_fid=fid
  if (fid eq -1) then begin
    envi_batch_exit
    return
  endif
  ;
  ; Open the file to read in array
  OpenR, Lun, fNameInput,/Get_Lun

  ; Setup the values for the keywords

  envi_file_query, fid, ns=ns, nl=nl, nb=nb, data_type=data_type,$
	               interleave=interleave, dims=dims

  Image=Make_Array(ns,nl,nb); create an array to get the values of the
image
  readu, lun, Image; read the image pixel values into the array
  Free_Lun, Lun

  ;print a pixel value by sample and rows
  print, image[1040, 404, *] ;this prints pixel values for 6 bands
  ; this fine, correct

  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;Retrieve a pixel value by geographic long lats
  ;get coordinates for image and convert to lat/lon
	  xcoord=116.521
	  ycoord= 40.1455

  ;change from lat/long coordinates to pixel values

  ENVI_CONVERT_File_COORDINATES, fid, xF, yF,xcoord, ycoord
  for nb=0, nb-1 do begin  ;read multi band data
      data = ENVI_GET_DATA(fid=fid, dims=dims, pos=nb); pos is the
name of the band, 0=1st band, 5=6th band
      output=data[xcoord, ycoord,*]
      print, output; this prints pixel values for 6 bands
  endfor
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;