diff --git a/filter/f-inst.c b/filter/f-inst.c
index 6c3724ad..6820b898 100644
--- a/filter/f-inst.c
+++ b/filter/f-inst.c
@@ -886,6 +886,8 @@
 		}]]);
 	}
       }
+      else if (da->empty)
+	RESULT_VAL(da->empty(da));
       else if ((empty = f_get_empty(da->type)).type != T_VOID)
 	RESULT_VAL(empty);
       else
diff --git a/filter/test.conf b/filter/test.conf
index 24094e80..39ec0edc 100644
--- a/filter/test.conf
+++ b/filter/test.conf
@@ -2301,9 +2301,9 @@ bool t;
 	krt_source = 17;
 	krt_metric = 19;
 
-#	krt_lock_mtu = false;
+#	if krt_lock_congctl then krt_lock_mtu = false;
 #	krt_lock_window = true;
-#	krt_lock_rtt = krt_lock_rttvar && krt_lock_sstresh || krt_lock_cwnd;
+#	krt_lock_rtt = krt_lock_rttvar && krt_lock_ssthresh || krt_lock_cwnd;
 
 	accept "ok I take that";
 }
diff --git a/lib/route.h b/lib/route.h
index 8d744e43..882e6fd4 100644
--- a/lib/route.h
+++ b/lib/route.h
@@ -278,6 +278,7 @@ struct ea_class {
   void (*format)(const eattr *ea, byte *buf, uint size); \
   void (*stored)(const eattr *ea);	/* When stored into global hash */ \
   void (*freed)(const eattr *ea);	/* When released from global hash */ \
+  struct f_val (*empty)(const struct ea_class *);	/* Return this instead of T_VOID as default value for filters */ \
 
   EA_CLASS_INSIDE;
 };
diff --git a/sysdep/linux/netlink.c b/sysdep/linux/netlink.c
index 6e10f543..ea79a1aa 100644
--- a/sysdep/linux/netlink.c
+++ b/sysdep/linux/netlink.c
@@ -51,6 +51,8 @@ struct nl_parse_state
 #define KRT_FEATURES_MAX	4
 
 static void krt_bitfield_format(const eattr *e, byte *buf, uint buflen);
+static struct f_val krt_bitfield_empty(const struct ea_class *cls UNUSED)
+{ return (struct f_val) { .type = T_INT }; }
 
 static struct ea_class
   ea_krt_prefsrc = {
@@ -71,11 +73,13 @@ static struct ea_class ea_krt_metrics[] = {
     .name = "krt_lock",
     .type = T_INT,
     .format = krt_bitfield_format,
+    .empty = krt_bitfield_empty,
   },
   [RTAX_FEATURES] = {
     .name = "krt_features",
     .type = T_INT,
     .format = krt_bitfield_format,
+    .empty = krt_bitfield_empty,
   },
   [RTAX_CC_ALGO] = {
     .name = "krt_congctl",