RUBY-165 raise error if BSON.serialize is passed something other than a hash

ext/cbson/cbson.c

@@ -562,8 +562,12 @@ static void write_doc(buffer_t buffer, VALUE hash, VALUE check_keys, VALUE move_
 
             write_function(key, value, pack_extra(buffer, check_keys));
         }
-    } else {
+    } else if (strcmp(rb_obj_classname(hash), "Hash") == 0) {
         rb_hash_foreach(hash, write_function, pack_extra(buffer, check_keys));
+    } else {
+        buffer_free(buffer);
+        char* cls = rb_obj_classname(hash);
+        rb_raise(InvalidDocument, "BSON.serialize takes a Hash but got a %s", cls);
     }
 
     // write null byte and fill in length

lib/bson/bson_ruby.rb

@@ -89,6 +89,7 @@ module BSON
     end
 
     def serialize(obj, check_keys=false, move_id=false)
+      raise(InvalidDocument, "BSON.serialize takes a Hash but got a #{obj.class}") unless obj.is_a?(Hash)
       raise "Document is null" unless obj
 
       @buf.rewind

test/mongo_bson/bson_test.rb

@@ -22,6 +22,20 @@ class BSONTest < Test::Unit::TestCase
 
   include BSON
 
+  def test_require_hash
+    assert_raise_error InvalidDocument, "takes a Hash" do
+      BSON.serialize('foo')
+    end
+
+    assert_raise_error InvalidDocument, "takes a Hash" do
+      BSON.serialize(Object.new)
+    end
+
+    assert_raise_error InvalidDocument, "takes a Hash" do
+      BSON.serialize(Set.new)
+    end
+  end
+
   def test_read_bson_io_document
     doc = {'doc' => 'hello, world'}
     bson = BSON.serialize(doc)