Used a couple "#if SUBPROTOCOL_VERSION != 0" conditionals.

compat.c

@@ -67,13 +67,19 @@ static void check_sub_protocol(void)
 {
 	char *dot;
 	int their_protocol, their_sub;
+#if SUBPROTOCOL_VERSION != 0
 	int our_sub = protocol_version < PROTOCOL_VERSION ? 0 : SUBPROTOCOL_VERSION;
+#else
+	int our_sub = 0;
+#endif
 
 	if (!shell_cmd || !(dot = strchr(shell_cmd, '.'))
 	 || !(their_protocol = atoi(shell_cmd))
 	 || !(their_sub = atoi(dot+1))) {
+#if SUBPROTOCOL_VERSION != 0
 		if (our_sub)
 			protocol_version--;
+#endif
 		return;
 	}
 
@@ -84,7 +90,7 @@ static void check_sub_protocol(void)
 	}
 
 	if (their_protocol > protocol_version)
-		their_sub = 0; /* 0 == final version */
+		their_sub = 0; /* 0 == final version of older protocol */
 	if (their_sub != our_sub)
 		protocol_version--;
 }