Failed trying to use the Windows dynamic library function AssocQueryStringA

In a Flutter application, I’m trying to use the function AssocQueryStringA to query some info about a file extension. By dart:ffi I’m using the following code to declare and load the function:

final _shlwapi = DynamicLibrary.open('shlwapi.dll');

static int assocQueryString(
      int flags,
      int str,
      Pointer<Utf16> pszAssoc,
      Pointer<Utf16> pszExtra,
      Pointer<Utf16> pszOut,
      Pointer<Uint32> pcchOut) =>
  _assocQueryString(flags, str, pszAssoc, pszExtra, pszOut, pcchOut);

static final _assocQueryString = _shlwapi.lookupFunction<
  Uint32 Function(
      IntPtr flags,
      IntPtr str,
      Pointer<Utf16> pszAssoc,
      Pointer<Utf16> pszExtra,
      Pointer<Utf16> pszOut,
      Pointer<Uint32> pcchOut),
  int Function(
      int flags,
      int str,
      Pointer<Utf16> pszAssoc,
      Pointer<Utf16> pszExtra,
      Pointer<Utf16> pszOut,
      Pointer<Uint32> pcchOut)>('AssocQueryStringA');

Then I call the function with this code, I passed ASSOCSTR_FRIENDLYAPPNAME with the value of 4 in the second argument:

Pointer<Utf16> temp = wsalloc(MAX_PATH);
Pointer<Uint32> tempSize = malloc<DWORD>();
assocQueryString(0, 4, TEXT(".xlsx"), nullptr, temp, tempSize);
var tt = temp.toDartString();

but the value returned in the tt variable is not readable as the following image show:

Is there something I’m doing wrong?