diff --git a/filter/f-inst.c b/filter/f-inst.c
index 4041a804..cee5b8e7 100644
--- a/filter/f-inst.c
+++ b/filter/f-inst.c
@@ -11,47 +11,35 @@
 
 /* Binary operators */
   case FI_ADD:
-    ARG(1,T_INT);
-    ARG(2,T_INT);
-    res.type = T_INT;
-    res.val.i = v1.val.i + v2.val.i;
+    ARG_T(1,0,T_INT);
+    ARG_T(2,1,T_INT);
+    res.val.i += v1.val.i;
     break;
   case FI_SUBTRACT:
-    ARG(1,T_INT);
-    ARG(2,T_INT);
-    res.type = T_INT;
-    res.val.i = v1.val.i - v2.val.i;
+    ARG_T(1,0,T_INT);
+    ARG_T(2,1,T_INT);
+    res.val.i -= v1.val.i;
     break;
   case FI_MULTIPLY:
-    ARG(1,T_INT);
-    ARG(2,T_INT);
-    res.type = T_INT;
-    res.val.i = v1.val.i * v2.val.i;
+    ARG_T(1,0,T_INT);
+    ARG_T(2,1,T_INT);
+    res.val.i *= v1.val.i;
     break;
   case FI_DIVIDE:
-    ARG(1,T_INT);
-    ARG(2,T_INT);
-    res.type = T_INT;
-    if (v2.val.i == 0) runtime( "Mother told me not to divide by 0" );
-    res.val.i = v1.val.i / v2.val.i;
+    ARG_T(1,0,T_INT);
+    ARG_T(2,1,T_INT);
+    if (v1.val.i == 0) runtime( "Mother told me not to divide by 0" );
+    res.val.i /= v1.val.i;
     break;
   case FI_AND:
-    ARG(1,T_BOOL);
-    if (!v1.val.i) {
-      res = v1;
-    } else {
-      ARG(2,T_BOOL);
-      res = v2;
-    }
+    ARG_T(1,0,T_BOOL);
+    if (res.val.i)
+      ARG_T(2,0,T_BOOL);
     break;
   case FI_OR:
-    ARG(1,T_BOOL);
-    if (v1.val.i) {
-      res = v1;
-    } else {
-      ARG(2,T_BOOL);
-      res = v2;
-    }
+    ARG_T(1,0,T_BOOL);
+    if (!res.val.i)
+      ARG_T(2,0,T_BOOL);
     break;
   case FI_PAIR_CONSTRUCT:
     ARG(1,T_INT);
@@ -184,8 +172,7 @@
     break;
 
   case FI_NOT:
-    ARG(1,T_BOOL);
-    res = v1;
+    ARG_T(1,0,T_BOOL);
     res.val.i = !res.val.i;
     break;
 
@@ -642,7 +629,6 @@
     break;
   case FI_ROUTE_DISTINGUISHER:
     ARG(1, T_NET);
-    res.type = T_IP;
     if (!net_is_vpn(v1.val.net))
       runtime( "VPN address expected" );
     res.type = T_RD;
@@ -671,11 +657,10 @@
     res.val.i = as_path_get_last_nonaggregated(v1.val.ad);
     break;
   case FI_RETURN:
-    ARG_ANY(1);
-    res = v1;
+    ARG_ANY_T(1,0);
     return F_RETURN;
   case FI_CALL:
-    ARG_ANY(1);
+    ARG_ANY_T(1,0);
     fret = interpret(fs, what->a2.p);
     if (fret > F_RETURN)
       return fret;
diff --git a/filter/filter.c b/filter/filter.c
index b15ede8a..a1bb7415 100644
--- a/filter/filter.c
+++ b/filter/filter.c
@@ -634,6 +634,7 @@ interpret(struct filter_state *fs, struct f_inst *what)
   u32 as;
 
 #define res fs->stack[fs->stack_ptr].val
+#define v0 res
 #define v1 fs->stack[fs->stack_ptr + 1].val
 #define v2 fs->stack[fs->stack_ptr + 2].val
 #define v3 fs->stack[fs->stack_ptr + 3].val
@@ -650,15 +651,18 @@ interpret(struct filter_state *fs, struct f_inst *what)
   return F_ERROR; \
 } while(0)
 
-#define ARG_ANY(n) INTERPRET(what->a##n.p, n)
+#define ARG_ANY_T(n, tt) INTERPRET(what->a##n.p, tt)
+#define ARG_ANY(n) ARG_ANY_T(n, n)
 
-#define ARG(n,t) do { \
-  ARG_ANY(n); \
-  if (v##n.type != t) \
+#define ARG_T(n,tt,t) do { \
+  ARG_ANY_T(n,tt); \
+  if (v##tt.type != t) \
     runtime("Argument %d of instruction %s must be of type %02x, got %02x", \
-	    n, f_instruction_name(what->fi_code), t, v##n.type); \
+	    n, f_instruction_name(what->fi_code), t, v##tt.type); \
 } while (0)
 
+#define ARG(n,t) ARG_T(n,n,t)
+
 #define INTERPRET(what_, n) do { \
   fs->stack_ptr += n; \
   fret = interpret(fs, what_); \