GetDiBits, где ошибка?

type
 RGB = packed record
   r: byte;
   g: byte;
   b: byte;
 end;
 TBitsmap = array of RGB;
var
 newBits : TBitsmap;
 oldBits : TBitsmap;

...

function GetBits(bmWork: TBitmap; var Bits : TBitsmap): boolean;
var
 bInfo : TBitmapInfo;
begin
 FillChar (bInfo, SizeOf(bInfo), 0);
 with bInfo.bmiHeader do
 begin
   biSize := SizeOf(bInfo);
   biWidth := wWidth;
   biHeight := -wHeight;
   biPlanes := 1;
   biBitCount := GetPixFormat(bmWork);
   biCompression :=  BI_RGB;
 end;
 GetDIBits(bmWork.Canvas.Handle,bmWork.Handle,0,wHeight,nil,bInfo,
             DIB_RGB_COLORS);
 SetLength(Bits,bInfo.bmiHeader.biSizeImage);
 Result := GetDIBits(bmWork.Canvas.Handle,bmWork.Handle,0,wHeight,Bits,bInfo,
             DIB_RGB_COLORS) > 0;
end;

if not GetBits(bmNew,newBits) then
     raise Exception.Create('Can''t create bits array from new bitmap'+
          #10#13+SysErrorMessage(GetLastError));

   if not GetBits(bmOld,oldBits) then
     raise Exception.Create('Can''t create bits array from old bitmap:'+
          #10#13+SysErrorMessage(GetLastError));