diff --git a/ChangeLog b/ChangeLog
index c10ca38196..33372b3eab 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Nov  4 02:06:16 2010  Yusuke Endoh  <mame@tsg.ne.jp>
+
+	* cont.c (fiber_t_alloc): raise an error when fiber is going to be
+	  initialized twice.  [ruby-dev:42524]
+
 Thu Nov  4 02:04:25 2010  Yusuke Endoh  <mame@tsg.ne.jp>
 
 	* cont.c (rb_fiber_resume): raise an "double resume" error when root
diff --git a/cont.c b/cont.c
index b0defcaeb7..56ae843d73 100644
--- a/cont.c
+++ b/cont.c
@@ -971,6 +971,10 @@ fiber_t_alloc(VALUE fibval)
     rb_fiber_t *fib;
     rb_thread_t *th = GET_THREAD();
 
+    if (DATA_PTR(fibval) != 0) {
+	rb_raise(rb_eRuntimeError, "cannot initialize twice");
+    }
+
     THREAD_MUST_BE_RUNNING(th);
     fib = ALLOC(rb_fiber_t);
     memset(fib, 0, sizeof(rb_fiber_t));