diff --git a/lib/idn.c b/lib/idn.c
index f35a4fd2e8..9483e734f0 100644
--- a/lib/idn.c
+++ b/lib/idn.c
@@ -152,7 +152,7 @@ static CURLcode mac_ascii_to_idn(const char *in, char **out)
 #ifdef USE_WIN32_IDN
 /* using Windows kernel32 and normaliz libraries. */
 
-#if !defined(_WIN32_WINNT) || _WIN32_WINNT < _WIN32_WINNT_VISTA
+#if !defined(_WIN32_WINNT) || _WIN32_WINNT < 0x600
 WINBASEAPI int WINAPI IdnToAscii(DWORD dwFlags,
                                  const WCHAR *lpUnicodeCharStr,
                                  int cchUnicodeChar,