diff --git a/src/main.c b/src/main.c
index cfff1f982..1c707f89f 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1764,6 +1764,10 @@ static int sockoptcallback(void *clientp, curl_socket_t curlfd,
           warnf(clientp, "Could not set TCP_KEEPINTVL!\n");
           return 0;
         }
+#endif
+#if !defined(TCP_KEEPIDLE) || !defined(TCP_KEEPINTVL)
+        warnf(clientp, "Keep-alive functionality somewhat crippled due to "
+              "missing support in your operating system!\n");
 #endif
       }
     }