diff --git a/libguile/programs.c b/libguile/programs.c
index 666b94fbf..122c1b776 100644
--- a/libguile/programs.c
+++ b/libguile/programs.c
@@ -90,6 +90,8 @@ SCM
 scm_c_make_closure (SCM program, SCM external)
 {
   SCM prog = scm_c_make_program (0, 0, program);
+  if (!SCM_PROGRAM_P (program))
+    abort ();
   *SCM_PROGRAM_DATA (prog) = *SCM_PROGRAM_DATA (program);
   SCM_PROGRAM_DATA (prog)->external = external;
   return prog;
diff --git a/libguile/vm-i-system.c b/libguile/vm-i-system.c
index 20a0b980d..16899cc59 100644
--- a/libguile/vm-i-system.c
+++ b/libguile/vm-i-system.c
@@ -64,6 +64,7 @@ VM_DEFINE_INSTRUCTION (halt, "halt", 0, 0, 0)
     {
       POP_LIST (nvalues);
       POP (ret);
+      SYNC_REGISTER ();
       ret = scm_values (ret);
     }
     
@@ -186,6 +187,7 @@ VM_DEFINE_INSTRUCTION (vector, "vector", 2, -1, 1)
   unsigned l = FETCH ();
   unsigned len = ((h << 8) + l);
   POP_LIST (len);
+  SYNC_REGISTER ();
   *sp = scm_vector (*sp);
   NEXT;
 }
@@ -199,6 +201,7 @@ VM_DEFINE_INSTRUCTION (list_mark, "list-mark", 0, 0, 0)
 VM_DEFINE_INSTRUCTION (vector_mark, "vector-mark", 0, 0, 0)
 {
   POP_LIST_MARK ();
+  SYNC_REGISTER ();
   *sp = scm_vector (*sp);
   NEXT;
 }
diff --git a/libguile/vm.c b/libguile/vm.c
index c14fb3b45..b93d7125e 100644
--- a/libguile/vm.c
+++ b/libguile/vm.c
@@ -331,7 +331,7 @@ vm_mark (SCM obj)
 
   /* mark the stack conservatively */
   scm_mark_locations ((SCM_STACKITEM *) vp->stack_base,
-                      vp->sp - vp->stack_base + 1);
+                      sizeof (SCM)*(vp->sp + 1 - vp->stack_base));
 
   /* mark other objects  */
   for (i = 0; i < SCM_VM_NUM_HOOKS; i++)