diff --git a/filter/decl.m4 b/filter/decl.m4
index 4d5b70dc..981d09a9 100644
--- a/filter/decl.m4
+++ b/filter/decl.m4
@@ -161,8 +161,12 @@ FID_HIC(,[[
 
 #	Some arguments need to check their type. After that, ARG_ANY is called.
 m4_define(ARG, `ARG_ANY($1)
+FID_NEW_BODY()m4_dnl
+if (f$1->type && (f$1->type != $2))
+  cf_error("Argument $1 of instruction %s must be of type $2, got 0x%02x", f_instruction_name(what->fi_code), f$1->type);
 FID_INTERPRET_EXEC()m4_dnl
-if (v$1.type != $2) runtime("Argument $1 of instruction %s must be of type $2, got 0x%02x", f_instruction_name(what->fi_code), v$1.type)m4_dnl
+if (v$1.type != $2)
+  runtime("Argument $1 of instruction %s must be of type $2, got 0x%02x", f_instruction_name(what->fi_code), v$1.type)m4_dnl
 FID_INTERPRET_BODY()')
 
 #	Executing another filter line. This replaces the recursion
@@ -202,11 +206,21 @@ FID_INTERPRET_BODY()')
 
 #	Some of the instructions have a result. These constructions
 #	state the result and put it to the right place.
-m4_define(RESULT, `RESULT_VAL([[ (struct f_val) { .type = $1, .val.$2 = $3 } ]])')
+m4_define(RESULT, `RESULT_TYPE([[$1]]) RESULT_([[$1]],[[$2]],[[$3]])')
+m4_define(RESULT_, `RESULT_VAL([[ (struct f_val) { .type = $1, .val.$2 = $3 } ]])')
 m4_define(RESULT_VAL, `FID_HIC(, [[do { res = $1; fstk->vcnt++; } while (0)]],
 [[return fi_constant(what, $1)]])')
 m4_define(RESULT_VOID, `RESULT_VAL([[ (struct f_val) { .type = T_VOID } ]])')
 
+m4_define(ERROR,
+       `m4_errprint(m4___file__:m4___line__: $*
+       )m4_m4exit(1)')
+
+m4_define(RESULT_TYPE,
+	`m4_ifdef([[INST_RESULT_TYPE]],
+		  [[m4_ifelse(INST_RESULT_TYPE,$1,,[[ERROR([[Multiple type definitons]])]])]],
+		  [[m4_define(INST_RESULT_TYPE,$1) FID_NEW_BODY()    what->type = $1;FID_INTERPRET_BODY()]])')
+
 #	Some common filter instruction members
 m4_define(SYMBOL, `FID_MEMBER(struct symbol *, sym, [[strcmp(f1->sym->name, f2->sym->name) || (f1->sym->class != f2->sym->class)]], "symbol %s", item->sym->name)')
 m4_define(RTC, `FID_MEMBER(struct rtable_config *, rtc, [[strcmp(f1->rtc->name, f2->rtc->name)]], "route table %s", item->rtc->name)')
@@ -343,8 +357,9 @@ m4_divert(-1)FID_FLUSH(101,200)m4_dnl  And finally this flushes all the unused d
 m4_define(INST, `m4_dnl				This macro is called on beginning of each instruction.
 INST_FLUSH()m4_dnl				First, old data is flushed
 m4_define([[INST_NAME]], [[$1]])m4_dnl		Then we store instruction name,
-m4_define([[INST_INVAL]], [[$2]])m4_dnl		instruction input value count
-m4_undefine([[INST_NEVER_CONSTANT]])m4_dnl	and reset NEVER_CONSTANT trigger.
+m4_define([[INST_INVAL]], [[$2]])m4_dnl		instruction input value count,
+m4_undefine([[INST_NEVER_CONSTANT]])m4_dnl	reset NEVER_CONSTANT trigger,
+m4_undefine([[INST_RESULT_TYPE]])m4_dnl		and reset RESULT_TYPE value.
 FID_INTERPRET_BODY()m4_dnl 			By default, every code is interpreter code.
 ')
 
@@ -541,6 +556,7 @@ FID_WR_PUT(4)m4_dnl
 struct f_inst {
   struct f_inst *next;			/* Next instruction */
   enum f_instruction_code fi_code;	/* Instruction code */
+  enum f_type type;			/* Type of returned value, if known */
   int size;				/* How many instructions are underneath */
   int lineno;				/* Line number */
   union {
diff --git a/filter/f-inst.c b/filter/f-inst.c
index 385d18d0..0bc48b3a 100644
--- a/filter/f-inst.c
+++ b/filter/f-inst.c
@@ -435,6 +435,7 @@
   INST(FI_VAR_GET, 0, 1) {
     SYMBOL;
     NEVER_CONSTANT;
+    RESULT_TYPE(sym->class & 0xff);
     RESULT_VAL(fstk->vstk[curline.vbase + sym->offset]);
   }
 
@@ -447,6 +448,7 @@
       val_dump(&(item->val))
     );
 
+    RESULT_TYPE(val.type);
     RESULT_VAL(val);
   }
 
@@ -588,31 +590,32 @@
     DYNAMIC_ATTR;
     ACCESS_RTE;
     ACCESS_EATTRS;
+    RESULT_TYPE(da.f_type);
     {
       eattr *e = ea_find(*fs->eattrs, da.ea_code);
 
       if (!e) {
 	/* A special case: undefined as_path looks like empty as_path */
 	if (da.type == EAF_TYPE_AS_PATH) {
-	  RESULT(T_PATH, ad, &null_adata);
+	  RESULT_(T_PATH, ad, &null_adata);
 	  break;
 	}
 
 	/* The same special case for int_set */
 	if (da.type == EAF_TYPE_INT_SET) {
-	  RESULT(T_CLIST, ad, &null_adata);
+	  RESULT_(T_CLIST, ad, &null_adata);
 	  break;
 	}
 
 	/* The same special case for ec_set */
 	if (da.type == EAF_TYPE_EC_SET) {
-	  RESULT(T_ECLIST, ad, &null_adata);
+	  RESULT_(T_ECLIST, ad, &null_adata);
 	  break;
 	}
 
 	/* The same special case for lc_set */
 	if (da.type == EAF_TYPE_LC_SET) {
-	  RESULT(T_LCLIST, ad, &null_adata);
+	  RESULT_(T_LCLIST, ad, &null_adata);
 	  break;
 	}
 
@@ -623,31 +626,31 @@
 
       switch (e->type & EAF_TYPE_MASK) {
       case EAF_TYPE_INT:
-	RESULT(da.f_type, i, e->u.data);
+	RESULT_(da.f_type, i, e->u.data);
 	break;
       case EAF_TYPE_ROUTER_ID:
-	RESULT(T_QUAD, i, e->u.data);
+	RESULT_(T_QUAD, i, e->u.data);
 	break;
       case EAF_TYPE_OPAQUE:
-	RESULT(T_ENUM_EMPTY, i, 0);
+	RESULT_(T_ENUM_EMPTY, i, 0);
 	break;
       case EAF_TYPE_IP_ADDRESS:
-	RESULT(T_IP, ip, *((ip_addr *) e->u.ptr->data));
+	RESULT_(T_IP, ip, *((ip_addr *) e->u.ptr->data));
 	break;
       case EAF_TYPE_AS_PATH:
-	RESULT(T_PATH, ad, e->u.ptr);
+	RESULT_(T_PATH, ad, e->u.ptr);
 	break;
       case EAF_TYPE_BITFIELD:
-	RESULT(T_BOOL, i, !!(e->u.data & (1u << da.bit)));
+	RESULT_(T_BOOL, i, !!(e->u.data & (1u << da.bit)));
 	break;
       case EAF_TYPE_INT_SET:
-	RESULT(T_CLIST, ad, e->u.ptr);
+	RESULT_(T_CLIST, ad, e->u.ptr);
 	break;
       case EAF_TYPE_EC_SET:
-	RESULT(T_ECLIST, ad, e->u.ptr);
+	RESULT_(T_ECLIST, ad, e->u.ptr);
 	break;
       case EAF_TYPE_LC_SET:
-	RESULT(T_LCLIST, ad, e->u.ptr);
+	RESULT_(T_LCLIST, ad, e->u.ptr);
 	break;
       case EAF_TYPE_UNDEF:
 	RESULT_VOID;
@@ -960,6 +963,8 @@
   INST(FI_CLIST_ADD, 2, 1) {	/* (Extended) Community list add */
     ARG_ANY(1);
     ARG_ANY(2);
+    RESULT_TYPE(f1->type);
+
     if (v1.type == T_PATH)
       runtime("Can't add to path");
 
@@ -969,14 +974,14 @@
       struct f_val dummy;
 
       if ((v2.type == T_PAIR) || (v2.type == T_QUAD))
-	RESULT(T_CLIST, ad, [[ int_set_add(fpool, v1.val.ad, v2.val.i) ]]);
+	RESULT_(T_CLIST, ad, [[ int_set_add(fpool, v1.val.ad, v2.val.i) ]]);
       /* IP->Quad implicit conversion */
       else if (val_is_ip4(&v2))
-	RESULT(T_CLIST, ad, [[ int_set_add(fpool, v1.val.ad, ipa_to_u32(v2.val.ip)) ]]);
+	RESULT_(T_CLIST, ad, [[ int_set_add(fpool, v1.val.ad, ipa_to_u32(v2.val.ip)) ]]);
       else if ((v2.type == T_SET) && clist_set_type(v2.val.t, &dummy))
 	runtime("Can't add set");
       else if (v2.type == T_CLIST)
-	RESULT(T_CLIST, ad, [[ int_set_union(fpool, v1.val.ad, v2.val.ad) ]]);
+	RESULT_(T_CLIST, ad, [[ int_set_union(fpool, v1.val.ad, v2.val.ad) ]]);
       else
 	runtime("Can't add non-pair");
     }
@@ -987,11 +992,11 @@
       if ((v2.type == T_SET) && eclist_set_type(v2.val.t))
 	runtime("Can't add set");
       else if (v2.type == T_ECLIST)
-	RESULT(T_ECLIST, ad, [[ ec_set_union(fpool, v1.val.ad, v2.val.ad) ]]);
+	RESULT_(T_ECLIST, ad, [[ ec_set_union(fpool, v1.val.ad, v2.val.ad) ]]);
       else if (v2.type != T_EC)
 	runtime("Can't add non-ec");
       else
-	RESULT(T_ECLIST, ad, [[ ec_set_add(fpool, v1.val.ad, v2.val.ec) ]]);
+	RESULT_(T_ECLIST, ad, [[ ec_set_add(fpool, v1.val.ad, v2.val.ec) ]]);
     }
 
     else if (v1.type == T_LCLIST)
@@ -1000,11 +1005,11 @@
       if ((v2.type == T_SET) && lclist_set_type(v2.val.t))
 	runtime("Can't add set");
       else if (v2.type == T_LCLIST)
-	RESULT(T_LCLIST, ad, [[ lc_set_union(fpool, v1.val.ad, v2.val.ad) ]]);
+	RESULT_(T_LCLIST, ad, [[ lc_set_union(fpool, v1.val.ad, v2.val.ad) ]]);
       else if (v2.type != T_LC)
 	runtime("Can't add non-lc");
       else
-	RESULT(T_LCLIST, ad, [[ lc_set_add(fpool, v1.val.ad, v2.val.lc) ]]);
+	RESULT_(T_LCLIST, ad, [[ lc_set_add(fpool, v1.val.ad, v2.val.lc) ]]);
 
     }
 
@@ -1015,6 +1020,8 @@
   INST(FI_CLIST_DEL, 2, 1) {	/* (Extended) Community list add or delete */
     ARG_ANY(1);
     ARG_ANY(2);
+    RESULT_TYPE(f1->type);
+
     if (v1.type == T_PATH)
     {
       const struct f_tree *set = NULL;
@@ -1027,7 +1034,7 @@
       else
 	runtime("Can't delete non-integer (set)");
 
-      RESULT(T_PATH, ad, [[ as_path_filter(fpool, v1.val.ad, set, key, 0) ]]);
+      RESULT_(T_PATH, ad, [[ as_path_filter(fpool, v1.val.ad, set, key, 0) ]]);
     }
 
     else if (v1.type == T_CLIST)
@@ -1036,12 +1043,12 @@
       struct f_val dummy;
 
       if ((v2.type == T_PAIR) || (v2.type == T_QUAD))
-	RESULT(T_CLIST, ad, [[ int_set_del(fpool, v1.val.ad, v2.val.i) ]]);
+	RESULT_(T_CLIST, ad, [[ int_set_del(fpool, v1.val.ad, v2.val.i) ]]);
       /* IP->Quad implicit conversion */
       else if (val_is_ip4(&v2))
-	RESULT(T_CLIST, ad, [[ int_set_del(fpool, v1.val.ad, ipa_to_u32(v2.val.ip)) ]]);
+	RESULT_(T_CLIST, ad, [[ int_set_del(fpool, v1.val.ad, ipa_to_u32(v2.val.ip)) ]]);
       else if ((v2.type == T_SET) && clist_set_type(v2.val.t, &dummy) || (v2.type == T_CLIST))
-	RESULT(T_CLIST, ad, [[ clist_filter(fpool, v1.val.ad, &v2, 0) ]]);
+	RESULT_(T_CLIST, ad, [[ clist_filter(fpool, v1.val.ad, &v2, 0) ]]);
       else
 	runtime("Can't delete non-pair");
     }
@@ -1050,22 +1057,22 @@
     {
       /* v2.val is either EC or EC-set */
       if ((v2.type == T_SET) && eclist_set_type(v2.val.t) || (v2.type == T_ECLIST))
-	RESULT(T_ECLIST, ad, [[ eclist_filter(fpool, v1.val.ad, &v2, 0) ]]);
+	RESULT_(T_ECLIST, ad, [[ eclist_filter(fpool, v1.val.ad, &v2, 0) ]]);
       else if (v2.type != T_EC)
 	runtime("Can't delete non-ec");
       else
-	RESULT(T_ECLIST, ad, [[ ec_set_del(fpool, v1.val.ad, v2.val.ec) ]]);
+	RESULT_(T_ECLIST, ad, [[ ec_set_del(fpool, v1.val.ad, v2.val.ec) ]]);
     }
 
     else if (v1.type == T_LCLIST)
     {
       /* v2.val is either LC or LC-set */
       if ((v2.type == T_SET) && lclist_set_type(v2.val.t) || (v2.type == T_LCLIST))
-	RESULT(T_LCLIST, ad, [[ lclist_filter(fpool, v1.val.ad, &v2, 0) ]]);
+	RESULT_(T_LCLIST, ad, [[ lclist_filter(fpool, v1.val.ad, &v2, 0) ]]);
       else if (v2.type != T_LC)
 	runtime("Can't delete non-lc");
       else
-	RESULT(T_LCLIST, ad, [[ lc_set_del(fpool, v1.val.ad, v2.val.lc) ]]);
+	RESULT_(T_LCLIST, ad, [[ lc_set_del(fpool, v1.val.ad, v2.val.lc) ]]);
     }
 
     else
@@ -1075,12 +1082,14 @@
   INST(FI_CLIST_FILTER, 2, 1) {	/* (Extended) Community list add or delete */
     ARG_ANY(1);
     ARG_ANY(2);
+    RESULT_TYPE(f1->type);
+
     if (v1.type == T_PATH)
     {
       u32 key = 0;
 
       if ((v2.type == T_SET) && (v2.val.t->from.type == T_INT))
-	RESULT(T_PATH, ad, [[ as_path_filter(fpool, v1.val.ad, v2.val.t, key, 1) ]]);
+	RESULT_(T_PATH, ad, [[ as_path_filter(fpool, v1.val.ad, v2.val.t, key, 1) ]]);
       else
 	runtime("Can't filter integer");
     }
@@ -1091,7 +1100,7 @@
       struct f_val dummy;
 
       if ((v2.type == T_SET) && clist_set_type(v2.val.t, &dummy) || (v2.type == T_CLIST))
-	RESULT(T_CLIST, ad, [[ clist_filter(fpool, v1.val.ad, &v2, 1) ]]);
+	RESULT_(T_CLIST, ad, [[ clist_filter(fpool, v1.val.ad, &v2, 1) ]]);
       else
 	runtime("Can't filter pair");
     }
@@ -1100,7 +1109,7 @@
     {
       /* v2.val is either EC or EC-set */
       if ((v2.type == T_SET) && eclist_set_type(v2.val.t) || (v2.type == T_ECLIST))
-	RESULT(T_ECLIST, ad, [[ eclist_filter(fpool, v1.val.ad, &v2, 1) ]]);
+	RESULT_(T_ECLIST, ad, [[ eclist_filter(fpool, v1.val.ad, &v2, 1) ]]);
       else
 	runtime("Can't filter ec");
     }
@@ -1109,7 +1118,7 @@
     {
       /* v2.val is either LC or LC-set */
       if ((v2.type == T_SET) && lclist_set_type(v2.val.t) || (v2.type == T_LCLIST))
-	RESULT(T_LCLIST, ad, [[ lclist_filter(fpool, v1.val.ad, &v2, 1) ]]);
+	RESULT_(T_LCLIST, ad, [[ lclist_filter(fpool, v1.val.ad, &v2, 1) ]]);
       else
 	runtime("Can't filter lc");
     }
diff --git a/filter/test.conf b/filter/test.conf
index b1342819..7152ee72 100644
--- a/filter/test.conf
+++ b/filter/test.conf
@@ -63,8 +63,8 @@ bool b;
 	bt_assert(! false && ! false && true);
 	bt_assert(1 < 2 && 1 != 3);
 	bt_assert(true && true && ! false);
-	bt_assert(true || 1+"a");
-	bt_assert(!(false && 1+"a"));
+	# bt_assert(true || 1+"a");
+	# bt_assert(!(false && 1+"a"));
 	bt_assert(!(true && false));
 }