diff --git a/env/mock_env.cc b/env/mock_env.cc
index 8868b358d..66ecfeff3 100644
--- a/env/mock_env.cc
+++ b/env/mock_env.cc
@@ -379,7 +379,8 @@ class TestMemLogger : public Logger {
       gettimeofday(&now_tv, nullptr);
       const time_t seconds = now_tv.tv_sec;
       struct tm t;
-      localtime_r(&seconds, &t);
+      auto ret __attribute__((__unused__)) = localtime_r(&seconds, &t);
+      assert(ret);
       p += snprintf(p, limit - p,
                     "%04d/%02d/%02d-%02d:%02d:%02d.%06d ",
                     t.tm_year + 1900,