diff --git a/src/class.c b/src/class.c
index ade36fad6..af3ae4357 100644
--- a/src/class.c
+++ b/src/class.c
@@ -67,12 +67,9 @@ static void
 mt_rehash(mrb_state *mrb, mt_tbl *t)
 {
   int old_alloc = t->alloc;
-  int new_alloc = old_alloc+8;
+  int new_alloc = old_alloc > 0 ? old_alloc << 1 : 8;
   union mt_ptr *old_ptr = t->ptr;
 
-  khash_power2(new_alloc);
-  if (old_alloc == new_alloc) return;
-
   t->ptr = (union mt_ptr*)mrb_calloc(mrb, sizeof(union mt_ptr)+sizeof(mrb_sym), new_alloc);
   t->alloc = new_alloc;
   t->size = 0;
diff --git a/src/variable.c b/src/variable.c
index 4a373f33f..c213f4211 100644
--- a/src/variable.c
+++ b/src/variable.c
@@ -43,11 +43,9 @@ static void
 iv_rehash(mrb_state *mrb, iv_tbl *t)
 {
   int old_alloc = t->alloc;
-  int new_alloc = old_alloc+4;
+  int new_alloc = old_alloc > 0 ? old_alloc << 1 : 4;
   mrb_value *old_ptr = t->ptr;
 
-  khash_power2(new_alloc);
-
   t->ptr = (mrb_value*)mrb_calloc(mrb, sizeof(mrb_value)+sizeof(mrb_sym), new_alloc);
   t->size = 0;
   t->alloc = new_alloc;