diff --git a/vm_eval.c b/vm_eval.c
index ba57d90946..f128e8dd8e 100644
--- a/vm_eval.c
+++ b/vm_eval.c
@@ -243,7 +243,7 @@ add_empty_keyword(int *argc, const VALUE **argv, int *kw_splat)
             int n = *argc;
             VALUE v;
             VALUE *ptr;
-            ptr = rb_alloc_tmp_buffer2(&v, n, sizeof(VALUE));
+            ptr = rb_alloc_tmp_buffer2(&v, n+1, sizeof(VALUE));
             memcpy(ptr, *argv, sizeof(VALUE)*n);
             ptr[n] = rb_hash_new();
             *argc = ++n;