diff --git a/src/gc.c b/src/gc.c
index 40c85a373e..429edade8e 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -282,6 +282,7 @@ mrb_alloca(mrb_state *mrb, size_t size)
 {
   struct RString *s;
   s = (struct RString*)mrb_obj_alloc(mrb, MRB_TT_STRING, mrb->string_class);
+  s->as.heap.aux.capa = size;
   return s->as.heap.ptr = (char*)mrb_malloc(mrb, size);
 }