diff --git a/lib/src/HttpAppFrameworkImpl.h b/lib/src/HttpAppFrameworkImpl.h
index 41edffa0..c43c7f86 100644
--- a/lib/src/HttpAppFrameworkImpl.h
+++ b/lib/src/HttpAppFrameworkImpl.h
@@ -148,6 +148,6 @@ namespace drogon
         std::string _sslKeyPath;
 
         size_t _maxConnectionNum=100000;
-        std::atomic_uint64_t _connectionNum;
+        std::atomic<uint64_t> _connectionNum;
     };
-}
\ No newline at end of file
+}