diff --git a/src/state.c b/src/state.c
index 612e1ac6e..40affd1bf 100644
--- a/src/state.c
+++ b/src/state.c
@@ -199,6 +199,7 @@ mrb_str_pool(mrb_state *mrb, mrb_value str)
       ns->flags = 0;
       ns->as.heap.ptr = (char *)mrb_malloc(mrb, (size_t)len+1);
       ns->as.heap.len = len;
+      ns->as.heap.aux.capa = len;
       if (ptr) {
         memcpy(ns->as.heap.ptr, ptr, len);
       }