diff --git a/BUILD b/BUILD
index 9c3aed78eb..1294b60201 100644
--- a/BUILD
+++ b/BUILD
@@ -830,6 +830,7 @@ cc_test(
     name = "arena_test",
     srcs = ["upb/mem/arena_test.cc"],
     deps = [
+        ":port",
         ":upb",
         "@com_google_absl//absl/random",
         "@com_google_absl//absl/random:distributions",
diff --git a/upb/mem/arena_test.cc b/upb/mem/arena_test.cc
index 8355d7babc..b09199726d 100644
--- a/upb/mem/arena_test.cc
+++ b/upb/mem/arena_test.cc
@@ -12,6 +12,9 @@
 #include "absl/synchronization/notification.h"
 #include "upb/upb.hpp"
 
+// Must be last.
+#include "upb/port/def.inc"
+
 namespace {
 
 static void decrement_int(void* ptr) {