diff --git a/filter/data.c b/filter/data.c
index 7295887d..09c71e5a 100644
--- a/filter/data.c
+++ b/filter/data.c
@@ -79,6 +79,8 @@ f_type_element_type(enum f_type t)
   };
 }
 
+const struct f_trie f_const_empty_trie = { .ipv4 = -1, };
+
 const struct f_val f_const_empty_path = {
   .type = T_PATH,
   .val.ad = &null_adata,
@@ -91,6 +93,9 @@ const struct f_val f_const_empty_path = {
 }, f_const_empty_lclist = {
   .type = T_LCLIST,
   .val.ad = &null_adata,
+}, f_const_empty_prefix_set = {
+  .type = T_PREFIX_SET,
+  .val.ti = &f_const_empty_trie,
 };
 
 static struct adata *
@@ -187,6 +192,11 @@ val_compare(const struct f_val *v1, const struct f_val *v2)
     if (val_is_ip4(v1) && (v2->type == T_QUAD))
       return uint_cmp(ipa_to_u32(v1->val.ip), v2->val.i);
 
+    if ((v1->type == T_SET) && (v2->type == T_PREFIX_SET))
+      return (!v1->val.t) && (v2->val.ti == &f_const_empty_trie) ? 0 : F_CMP_ERROR;
+    if ((v1->type == T_PREFIX_SET) && (v2->type == T_SET))
+      return (v1->val.ti == &f_const_empty_trie) && (!v2->val.t) ? 0 : F_CMP_ERROR;
+
     debug( "Types do not match in val_compare\n" );
     return F_CMP_ERROR;
   }
diff --git a/filter/data.h b/filter/data.h
index 84d48a7c..221e4229 100644
--- a/filter/data.h
+++ b/filter/data.h
@@ -299,7 +299,7 @@ undef_value(struct f_val v)
     (v.val.ad == &null_adata);
 }
 
-extern const struct f_val f_const_empty_path, f_const_empty_clist, f_const_empty_eclist, f_const_empty_lclist;
+extern const struct f_val f_const_empty_path, f_const_empty_clist, f_const_empty_eclist, f_const_empty_lclist, f_const_empty_prefix_set;
 
 enum filter_return f_eval(const struct f_line *expr, struct linpool *tmp_pool, struct f_val *pres);
 
diff --git a/filter/decl.m4 b/filter/decl.m4
index 75e65212..24d831b3 100644
--- a/filter/decl.m4
+++ b/filter/decl.m4
@@ -497,10 +497,7 @@ f_const_promotion(struct f_inst *arg, enum f_type want)
   else if ((c->type == T_SET) && (want == T_PREFIX_SET)) {
     if (c->val.t)
       return 0;
-    *c = (struct f_val) {
-      .type = T_PREFIX_SET,
-      .val.ti = f_new_trie(cfg_mem, 0),
-    };
+    *c = f_const_empty_prefix_set;
     return 1;
   }
 
diff --git a/filter/f-inst.c b/filter/f-inst.c
index 999b7fc7..b0dce4f9 100644
--- a/filter/f-inst.c
+++ b/filter/f-inst.c
@@ -1282,14 +1282,12 @@
       u32 key = 0;
       int use_set = 0;
 
-      if (v2.type == T_INT)
-	key = v2.val.i;
-      else if ((v2.type == T_SET) && path_set_type(v2.val.t)) {
+      if ((v2.type == T_SET) && path_set_type(v2.val.t)) {
         use_set = 1;
         set = v2.val.t;
       }
       else
-	runtime("Can't filter non-integer (set)");
+	runtime("Can't filter integer");
 
       RESULT_(T_PATH, ad, [[ as_path_filter(fpool, v1.val.ad, set, key, use_set, 1) ]]);
     }