Application Error ved skrivning til fil

Hej!

Min kode generere en application Error når den fil jeg skriver til lukkes til aller sidst i proceduren. Men kun hvis den fil jeg henter dataen fra er stører en min read/write-buffer (buf). Hvordan kan jeg undgå det?

procedure TForm1.EncryptFile1Click(Sender: TObject);
var
  numRead, numWritten,i, charNum, codeCount, counter, enum, codeNum: integer;
  buf, outbuf : array[1..2048] of Char;
  currentChar : char;

begin
  if length(codeText.Text) > 0 then
  if SD1.Execute then
  begin
    assignFile(toF,SD1.FileName);
    rewrite(toF,1);
    begin
      if OD1.Execute then
      begin
        assignFile(fromF, OD1.FileName);
        reset(fromF,1);
      end
      else
      begin
        reset(fromF,1);
      end;
      Repeat
        predefine;
        BlockRead(fromF, buf, SizeOf(buf), NumRead);
        begin
          codeCount := 1;
          for i := 0 to length(buf) do
          if inArray(buf[i]) then
          begin
            charNum := lord(buf[i]);
            if codeCount = length(code) then
            begin
              codeCount := 1;
            end
            else
            begin
              codeCount := codeCount+1;
            end;
            codeNum := ord(code[codeCount])+round((power(sin(codeCount)*10,3)/4)+(100-codeCount));
            enum := charNum+codeNum;
            outbuf[i] := lchar(enum);
          end
          else
          begin
            outbuf[i] := buf[i];
          end;
        end;
        blockWrite(ToF, outBuf, NumRead, NumWritten);
      until (NumRead = 0) or (NumWritten <> NumRead);
      CloseFile(fromF);
      CloseFile(toF);
    end;
  end;
end;