diff --git a/lib/idn.c b/lib/idn.c
index 9483e734f0..2d56043293 100644
--- a/lib/idn.c
+++ b/lib/idn.c
@@ -152,7 +152,8 @@ 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 < 0x600
+#if (!defined(_WIN32_WINNT) || _WIN32_WINNT < _WIN32_WINNT_VISTA) && \
+  (!defined(WINVER) || WINVER < 0x600)
 WINBASEAPI int WINAPI IdnToAscii(DWORD dwFlags,
                                  const WCHAR *lpUnicodeCharStr,
                                  int cchUnicodeChar,