diff --git a/src/proto.h b/src/proto.h
index 7541ce32c1..4c0db37a1e 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -120,7 +120,9 @@ int
 _RTLENTRYF
 #  endif
 vim_snprintf __ARGS((char *, size_t, char *, ...));
+#  if defined(HAVE_STDARG_H)
 int vim_vsnprintf(char *str, size_t str_m, char *fmt, va_list ap, typval_T *tvs);
+#  endif
 # endif
 
 # include "message.pro"