diff --git a/bn_mp_init_copy.c b/bn_mp_init_copy.c
index ed58c16..ab87d23 100644
--- a/bn_mp_init_copy.c
+++ b/bn_mp_init_copy.c
@@ -20,7 +20,7 @@ int mp_init_copy (mp_int * a, mp_int * b)
 {
   int     res;
 
-  if ((res = mp_init (a)) != MP_OKAY) {
+  if ((res = mp_init_size (a, b->used)) != MP_OKAY) {
     return res;
   }
   return mp_copy (b, a);