Index: io.c
===================================================================
--- io.c	(revision 29690)
+++ io.c	(working copy)
@@ -2724,6 +2724,8 @@
     long limit;
 
     prepare_getline_args(argc, argv, &rs, &limit, io);
+    if (limit == 0) 
+	rb_raise(rb_eArgError, "invalid limit: 0 for readlines");
     ary = rb_ary_new();
     while (!NIL_P(line = rb_io_getline_1(rs, limit, io))) {
 	rb_ary_push(ary, line);
@@ -2773,6 +2775,8 @@
 
     RETURN_ENUMERATOR(io, argc, argv);
     prepare_getline_args(argc, argv, &rs, &limit, io);
+    if (limit == 0) 
+	rb_raise(rb_eArgError, "invalid limit: 0 for each_line");
     while (!NIL_P(str = rb_io_getline_1(rs, limit, io))) {
 	rb_yield(str);
     }
Index: test/ruby/test_io.rb
===================================================================
--- test/ruby/test_io.rb	(revision 29690)
+++ test/ruby/test_io.rb	(working copy)
@@ -1702,4 +1702,24 @@
       GC.start
     end
   end
+
+  def test_readlines_limit_0
+    bugxxx = "[ruby-dev:xxx]"
+    t = make_tempfile
+    assert_raise(ArgumentError, bugxxx) do
+      open(t.path, "r") do |io|
+        io.readlines(0)
+      end
+    end
+  end
+
+  def test_each_line_limit_0
+    bugxxx = "[ruby-dev:xxx]"
+    t = make_tempfile
+    assert_raise(ArgumentError, bugxxx) do
+      open(t.path, "r") do |io|
+        io.each_line(0).next
+      end
+    end
+  end
 end