diff --git a/lib/tds/parameter.ex b/lib/tds/parameter.ex
index 6de82b1..217c303 100644
--- a/lib/tds/parameter.ex
+++ b/lib/tds/parameter.ex
@@ -84,7 +84,7 @@ defmodule Tds.Parameter do
   def fix_data_type(%__MODULE__{type: nil, value: nil} = param) do
     # should fix ecto has_one, on_change :nulify issue where type is not know when ecto
     # build query/statement for on_chage callback
-    %{param | type: :string}
+    %{param | type: :binary}
   end
 
   def fix_data_type(%__MODULE__{value: value} = param)