diff --git a/src/pmc/bkmarshal.c b/src/pmc/bkmarshal.c
index 611ce59..9e96a72 100644
--- a/src/pmc/bkmarshal.c
+++ b/src/pmc/bkmarshal.c
@@ -71,7 +71,7 @@ blizkost_marshal_arg(BLIZKOST_NEXUS, PMC *arg) {
         char *c_str = Parrot_str_to_cstring(interp, VTABLE_get_string(interp, arg));
         result = sv_2mortal(newSVpv(c_str, strlen(c_str)));
     }
-    else if (VTABLE_isa(interp, arg, CONST_STRING(interp, "Sub"))) {
+    else if (VTABLE_does(interp, arg, CONST_STRING(interp, "invokable"))) {
         CV *wrapper = blizkost_wrap_callable(nexus, arg);
         result = sv_2mortal(newRV_inc((SV*)wrapper));
     }
@@ -316,11 +316,11 @@ XS(blizkost_callable_trampoline)
     SP -= items;
     PUTBACK;
 
-    args = Parrot_pmc_new(nexus->parrot_interp, enum_class_ResizablePMCArray);
+    args = Parrot_pmc_new_init_int(nexus->parrot_interp, enum_class_ResizablePMCArray, items);
     for (i = 0; i < items; i++) {
         SV *svarg = ST(i);
         PMC *pmcarg = blizkost_wrap_sv(nexus, svarg);
-        VTABLE_unshift_pmc(nexus->parrot_interp, args, pmcarg);
+        VTABLE_set_pmc_keyed_int(nexus->parrot_interp, args, i, pmcarg);
     }
 
     Parrot_pcc_invoke_sub_from_c_args(nexus->parrot_interp, callable,