Index: eval.c
===================================================================
--- eval.c	(revision 24590)
+++ eval.c	(working copy)
@@ -2854,9 +2854,11 @@
     if (NIL_P(tmp)) {
 	/* hack to avoid invoke Object#to_a */
 	VALUE origin;
+	const NODE *body;
 	ID id = rb_intern("to_a");
 
-	if (search_method(CLASS_OF(val), id, &origin) &&
+	if ((body = search_method(CLASS_OF(val), id, &origin)) &&
+	    body->nd_body &&  /* not an undef'd method */
 	    RCLASS(origin)->m_tbl != RCLASS(rb_mKernel)->m_tbl) { /* exclude Kernel#to_a */
 	    val = rb_funcall(val, id, 0);
 	    if (TYPE(val) != T_ARRAY) {