diff --git a/lib/mongo/collection.rb b/lib/mongo/collection.rb
index d717ed2..9a913a2 100644
--- a/lib/mongo/collection.rb
+++ b/lib/mongo/collection.rb
@@ -33,8 +33,8 @@ module Mongo
       if name.empty? or name.include? ".."
         raise InvalidName, "collection names cannot be empty"
       end
-      if name.include? "$" and not name.match(/^\$cmd/)
-        raise InvalidName, "collection names must not contain '$'"
+      if name.include? "$"
+        raise InvalidName, "collection names must not contain '$'" unless name =~ /((^\$cmd)|(oplog\.\$main))/
       end
       if name.match(/^\./) or name.match(/\.$/)
         raise InvalidName, "collection names must not start or end with '.'"
diff --git a/test/test_collection.rb b/test/test_collection.rb
index 59130ad..bc0eea5 100644
--- a/test/test_collection.rb
+++ b/test/test_collection.rb
@@ -26,11 +26,20 @@ class TestCollection < Test::Unit::TestCase
     assert @coll.pk_factory.is_a?(Object)
   end
 
-  def test_collection
+  def test_valid_names
     assert_raise InvalidName do
       @@db["te$t"]
     end
 
+    assert_raise InvalidName do
+      @@db['$main']
+    end
+
+    assert @@db['$cmd']
+    assert @@db['oplog.$main']
+  end
+
+  def test_collection
     assert_kind_of Collection, @@db["test"]
     assert_equal @@db["test"].name(), @@db.collection("test").name()
     assert_equal @@db["test"].name(), @@db[:test].name()