diff --git a/setup.h.in b/setup.h.in
index cf035834e6..3c2e778e31 100644
--- a/setup.h.in
+++ b/setup.h.in
@@ -1035,6 +1035,9 @@
 /* The type of 3rd argument to getsockname() - usually size_t or int */
 #undef SOCKLEN_T
 
+/* The type of 5th argument to getsockopt() - usually size_t or int */
+#undef SOCKOPTLEN_T
+
 /* The type of statvfs(2) argument */
 #undef WX_STATFS_T