diff --git a/include/mruby/internal.h b/include/mruby/internal.h
index 2e411ce6e..02e85d656 100644
--- a/include/mruby/internal.h
+++ b/include/mruby/internal.h
@@ -183,6 +183,8 @@ mrb_value mrb_bint_new_str(mrb_state *mrb, const char *x, mrb_int len, mrb_int b
 mrb_value mrb_as_bint(mrb_state *mrb, mrb_value x);
 mrb_value mrb_bint_add(mrb_state *mrb, mrb_value x, mrb_value y);
 mrb_value mrb_bint_sub(mrb_state *mrb, mrb_value x, mrb_value y);
+mrb_value mrb_bint_add_d(mrb_state *mrb, mrb_value x, mrb_value y);
+mrb_value mrb_bint_sub_d(mrb_state *mrb, mrb_value x, mrb_value y);
 mrb_value mrb_bint_mul(mrb_state *mrb, mrb_value x, mrb_value y);
 mrb_value mrb_bint_div(mrb_state *mrb, mrb_value x, mrb_value y);
 mrb_value mrb_bint_divmod(mrb_state *mrb, mrb_value x, mrb_value y);
diff --git a/mrbgems/mruby-bigint/core/bigint.c b/mrbgems/mruby-bigint/core/bigint.c
index 7ba9b1adf..3af5b2da9 100644
--- a/mrbgems/mruby-bigint/core/bigint.c
+++ b/mrbgems/mruby-bigint/core/bigint.c
@@ -1179,8 +1179,9 @@ mrb_bint_as_uint64(mrb_state *mrb, mrb_value x)
   return u;
 }
 
+/* unnormalize version of mrb_bint_add */
 mrb_value
-mrb_bint_add(mrb_state *mrb, mrb_value x, mrb_value y)
+mrb_bint_add_d(mrb_state *mrb, mrb_value x, mrb_value y)
 {
 #ifndef MRB_NO_FLOAT
   if (mrb_float_p(y)) {
@@ -1194,11 +1195,19 @@ mrb_bint_add(mrb_state *mrb, mrb_value x, mrb_value y)
   struct RBigint *b2 = RBIGINT(y);
   struct RBigint *b3 = bint_new(mrb);
   mpz_add(mrb, &b3->mp, &b->mp, &b2->mp);
-  return bint_norm(mrb, b3);
+  return mrb_obj_value(b3);
 }
 
 mrb_value
-mrb_bint_sub(mrb_state *mrb, mrb_value x, mrb_value y)
+mrb_bint_add(mrb_state *mrb, mrb_value x, mrb_value y)
+{
+  x = mrb_bint_add_d(mrb, x, y);
+  return bint_norm(mrb, RBIGINT(x));
+}
+
+/* unnormalize version of mrb_bint_sub */
+mrb_value
+mrb_bint_sub_d(mrb_state *mrb, mrb_value x, mrb_value y)
 {
 #ifndef MRB_NO_FLOAT
   if (mrb_float_p(y)) {
@@ -1212,7 +1221,14 @@ mrb_bint_sub(mrb_state *mrb, mrb_value x, mrb_value y)
   struct RBigint *b2 = RBIGINT(y);
   struct RBigint *b3 = bint_new(mrb);
   mpz_sub(mrb, &b3->mp, &b->mp, &b2->mp);
-  return bint_norm(mrb, b3);
+  return mrb_obj_value(b3);
+}
+
+mrb_value
+mrb_bint_sub(mrb_state *mrb, mrb_value x, mrb_value y)
+{
+  x = mrb_bint_sub_d(mrb, x, y);
+  return bint_norm(mrb, RBIGINT(x));
 }
 
 mrb_value