diff --git a/winbuild/MakefileBuild.vc b/winbuild/MakefileBuild.vc
index 5ec638f85..800bebcce 100644
--- a/winbuild/MakefileBuild.vc
+++ b/winbuild/MakefileBuild.vc
@@ -172,7 +172,7 @@ USE_IDN   = true
 !ENDIF
 
 !IF "$(USE_IDN)"=="true"
-CFLAGS    = $(CFLAGS) /DUSE_WIN32_IDN
+CFLAGS    = $(CFLAGS) /DUSE_WIN32_IDN /DWANT_IDN_PROTOTYPES
 LFLAGS    = $(LFLAGS) Normaliz.lib
 !ENDIF