diff --git a/acconfig.h b/acconfig.h
index 48020843b..86bc37d89 100644
--- a/acconfig.h
+++ b/acconfig.h
@@ -6,3 +6,5 @@
 
 #undef NeXT_cc
 #undef NeXT_runtime
+
+#undef VSPRINTF_RETURNS_LENGTH