What am I doing wrong with _FreeImage?

[PhilOfPerth]

I have written a small test using _loadimage and _FreeImage, and all goes well except when I try to clear the images (as I believe is necessary after using them).
I get an Illegal function call message at that point. Why is it so???

Code: (Select All)

Screen _NewImage(1500, 800, 32)

GetGridSize:
Locate 15, 66
Print "Choose a grid size, 1 to 4 (for 12, 20, 30 or 42 tiles)"
Play move$
Getsize:
_KeyClear: k = 0
While k < 1
    _Limit 30
    k = _KeyHit
Wend
Select Case k
    Case Is = 49
        numtiles = 12 '                                                                      numtiles is number of tiles for that size grid
        numcols = 3 '                                                                        numcols is number of columns in the grid
    Case Is = 50
        numtiles = 20
        numcols = 5
    Case Is = 51
        numtiles = 30
        numcols = 5
    Case Is = 52
        numtiles = 42
        numcols = 7
    Case Else
        GoTo Getsize
End Select

DisplayTiles: '
numrows = numtiles / numcols '                                                                set number of rows needed for the numtiles and numcols
Dim tiles(numtiles) As Long
For a = 1 To numtiles
    tiles(a) = _LoadImage("RecPics/test.jpg", 32) '                                          set tiles array with numpics copies of test.jpg
Next
For a = 1 To numtiles / 2
    _PutImage (60 * a, 60), tiles(a) '                                                      display first half of tiles  array
Next
For a = numtiles / 2 + 1 To numtiles
    _PutImage (60 * (a - numtiles / 2), 120), tiles(a) '                                    display second half of tiles array
Next
Sleep 2

For a = 1 To numtiles
    _FreeImage (a) '                                                                            free all of the images from memory
Next
Print "I get an error message here: Illegal function call line 45 (the _FreeImage line)"
Sleep