IEShowSaveFileDialog - Translation issues

Hoping someone can help me out. I can't seem to get the IEShowSaveFileDialog
function to work. It's either an error in my function translation or in the
way I'm calling it.

Here's the original C++ from iepmapi.h
STDAPI IEShowSaveFileDialog(
    __in        HWND     hwnd,
    __in        LPCWSTR  lpwstrInitialFileName,
    __in_opt    LPCWSTR  lpwstrInitialDir,
    __in_opt    LPCWSTR  lpwstrFilter,
    __in_opt    LPCWSTR  lpwstrDefExt,
    __in        DWORD    dwFilterIndex,
    __in        DWORD    dwFlags,
    __deref_out LPWSTR   *lppwstrDestinationFilePath,
    __out       HANDLE   *phState
    );

Here's my translation of it:
function IEShowSaveFileDialog(WinHandle : HWND; strInitialFileName : LPWSTR;
strInitialDir : LPWSTR;
       strFilter : LPCWSTR; strDefExt : LPCWSTR; dwFilterIndex : DWORD;
dwFlag : DWORD;
       strDestinationFilePath : pointer; var hState : THandle) : HRESULT;
stdcall; external 'ieframe.dll';

And here's the function call I'm using to test it (this is in an ActiveX
control):

procedure showtest;
var
  sDestFile : PWideChar;
  hState : THandle;
  res : HRESULT;
  dwSaveFlags : DWORD;
begin
  dwSaveFlags := OFN_DONTADDTORECENT or OFN_NOCHANGEDIR;
  res := IEShowSaveFileDialog(Self.Handle,'test.txt',nil,'text
files|*.txt','txt',1,dwSaveFlags,sDestFile,hState);
end;

The result (res) is 80070057, which is Invalid Argument.

Thanks for reading,
Chris