diff --git a/test/test-threadpool-cancel.c b/test/test-threadpool-cancel.c
index 917f5f47..dd13d8ae 100644
--- a/test/test-threadpool-cancel.c
+++ b/test/test-threadpool-cancel.c
@@ -60,7 +60,10 @@ static void saturate_threadpool(void) {
   char buf[64];
   size_t i;
 
-  snprintf(buf, sizeof(buf), "UV_THREADPOOL_SIZE=%zu", ARRAY_SIZE(pause_reqs));
+  snprintf(buf,
+           sizeof(buf),
+           "UV_THREADPOOL_SIZE=%lu",
+           (unsigned long)ARRAY_SIZE(pause_reqs));
   putenv(buf);
 
   loop = uv_default_loop();