diff --git a/ChangeLog b/ChangeLog
index b874507208..4c0444fd46 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Dec 21 14:58:27 2007  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* enumerator.c (enumerator_init_copy): prohibit cloning of
+	  generators since Fibers cannot be copied.
+
 Fri Dec 21 14:46:07 2007  Tanaka Akira  <akr@fsij.org>
 
 	* io.c (Init_IO): define IO::BINARY even if O_BINARY is not exist.
diff --git a/enumerator.c b/enumerator.c
index 366c64c28a..729d7c04ca 100644
--- a/enumerator.c
+++ b/enumerator.c
@@ -285,14 +285,20 @@ enumerator_initialize(int argc, VALUE *argv, VALUE obj)
 static VALUE
 enumerator_init_copy(VALUE obj, VALUE orig)
 {
-    struct enumerator *ptr0 = enumerator_ptr(orig);
-    struct enumerator *ptr1 = enumerator_ptr(obj);
+    struct enumerator *ptr0, *ptr1;
+
+    ptr0 = enumerator_ptr(orig);
+    if (ptr1->fib) {
+	/* Fibers cannot be copied */
+	rb_raise(rb_eTypeError, "can't copy execution context");
+    }
+    ptr1 = enumerator_ptr(obj);
 
     ptr1->method = ptr0->method;
     ptr1->proc = ptr0->proc;
     ptr1->iter = ptr0->iter;
     ptr1->args = ptr0->args;
-    ptr1->fib  = ptr0->fib;
+    ptr1->fib  = 0;
 
     return obj;
 }