diff --git a/src/vm_scheme.c b/src/vm_scheme.c
index 4b133a259..3ab57f694 100644
--- a/src/vm_scheme.c
+++ b/src/vm_scheme.c
@@ -52,7 +52,7 @@ SCM_DEFINE_VM_FUNCTION (null_p, "null?", "null?", 1, 0)
 SCM_DEFINE_VM_FUNCTION (cons, "cons", "cons", 2, 0)
 {
   VM_SETUP_ARGS2 ();
-  CONS (ac, ac, a2);
+  CONS (ac, a1, a2);
   NEXT;
 }
 
diff --git a/src/vm_system.c b/src/vm_system.c
index 47688fa27..7d8d18282 100644
--- a/src/vm_system.c
+++ b/src/vm_system.c
@@ -372,7 +372,10 @@ SCM_DEFINE_INSTRUCTION (br_if_null, "%br-if-null", INST_ADDR)
 {
   SCM addr = FETCH (); /* must always fetch */
   if (SCM_NULLP (ac))
-    pc = SCM_VM_ADDRESS (addr);
+    {
+      ac = SCM_BOOL_T;
+      pc = SCM_VM_ADDRESS (addr);
+    }
   NEXT;
 }