diff --git a/ext/cbson/cbson.c b/ext/cbson/cbson.c
index c89868f..9791d7a 100644
--- a/ext/cbson/cbson.c
+++ b/ext/cbson/cbson.c
@@ -231,12 +231,12 @@ static int write_element(VALUE key, VALUE value, VALUE extra, int allow_id) {
         int i;
         if (RSTRING_LEN(key) > 0 && RSTRING_PTR(key)[0] == '$') {
             buffer_free(buffer);
-            rb_raise(InvalidKeyName, "key must not start with '$'");
+            rb_raise(InvalidKeyName, "%s - key must not start with '$'", RSTRING_PTR(key));
         }
         for (i = 0; i < RSTRING_LEN(key); i++) {
             if (RSTRING_PTR(key)[i] == '.') {
                 buffer_free(buffer);
-                rb_raise(InvalidKeyName, "key must not contain '.'");
+                rb_raise(InvalidKeyName, "%s - key must not contain '.'", RSTRING_PTR(key));
             }
         }
     }
diff --git a/test/bson/bson_test.rb b/test/bson/bson_test.rb
index 2b3a7a6..bf652d7 100644
--- a/test/bson/bson_test.rb
+++ b/test/bson/bson_test.rb
@@ -562,4 +562,47 @@ class BSONTest < Test::Unit::TestCase
     end
   end
 
+  def test_invalid_key_names
+    assert @encoder.serialize({"hello" => "world"}, true)
+    assert @encoder.serialize({"hello" => {"hello" => "world"}}, true)
+
+    assert @encoder.serialize({"he$llo" => "world"}, true)
+    assert @encoder.serialize({"hello" => {"hell$o" => "world"}}, true)
+
+    assert_raise BSON::InvalidDocument do
+      @encoder.serialize({"he\0llo" => "world"}, true)
+    end
+
+    assert_raise_error BSON::InvalidKeyName, "$hello" do
+      @encoder.serialize({"$hello" => "world"}, true)
+    end
+
+    assert_raise BSON::InvalidKeyName do
+      @encoder.serialize({"hello" => {"$hello" => "world"}}, true)
+    end
+
+    assert_raise_error BSON::InvalidKeyName, ".hello" do
+      @encoder.serialize({".hello" => "world"}, true)
+    end
+
+    assert_raise BSON::InvalidKeyName do
+      @encoder.serialize({"hello" => {".hello" => "world"}}, true)
+    end
+
+    assert_raise BSON::InvalidKeyName do
+      @encoder.serialize({"hello." => "world"}, true)
+    end
+
+    assert_raise BSON::InvalidKeyName do
+      @encoder.serialize({"hello" => {"hello." => "world"}}, true)
+    end
+
+    assert_raise BSON::InvalidKeyName do
+      @encoder.serialize({"hel.lo" => "world"}, true)
+    end
+
+    assert_raise BSON::InvalidKeyName do
+      @encoder.serialize({"hello" => {"hel.lo" => "world"}}, true)
+    end
+  end
 end
diff --git a/test/db_api_test.rb b/test/db_api_test.rb
index ecb5fb5..d1688a3 100644
--- a/test/db_api_test.rb
+++ b/test/db_api_test.rb
@@ -621,43 +621,6 @@ class DBAPITest < Test::Unit::TestCase
     assert_equal("mike", @@coll.find_one()["hello"])
   end
 
-  def test_invalid_key_names
-    @@coll.remove
-
-    @@coll.insert({"hello" => "world"})
-    @@coll.insert({"hello" => {"hello" => "world"}})
-
-    assert_raise BSON::InvalidKeyName do
-      @@coll.insert({"$hello" => "world"})
-    end
-
-    assert_raise BSON::InvalidKeyName do
-      @@coll.insert({"hello" => {"$hello" => "world"}})
-    end
-
-    @@coll.insert({"he$llo" => "world"})
-    @@coll.insert({"hello" => {"hell$o" => "world"}})
-
-    assert_raise BSON::InvalidKeyName do
-      @@coll.insert({".hello" => "world"})
-    end
-    assert_raise BSON::InvalidKeyName do
-      @@coll.insert({"hello" => {".hello" => "world"}})
-    end
-    assert_raise BSON::InvalidKeyName do
-      @@coll.insert({"hello." => "world"})
-    end
-    assert_raise BSON::InvalidKeyName do
-      @@coll.insert({"hello" => {"hello." => "world"}})
-    end
-    assert_raise BSON::InvalidKeyName do
-      @@coll.insert({"hel.lo" => "world"})
-    end
-    assert_raise BSON::InvalidKeyName do
-      @@coll.insert({"hello" => {"hel.lo" => "world"}})
-    end
-  end
-
   def test_collection_names
     assert_raise TypeError do
       @@db.collection(5)