diff --git a/docs/gmock_cook_book.md b/docs/gmock_cook_book.md
index de3464df..208ca9a6 100644
--- a/docs/gmock_cook_book.md
+++ b/docs/gmock_cook_book.md
@@ -3627,6 +3627,8 @@ as adding templates in the right place.
 
 class NotNullMatcher {
  public:
+  using is_gtest_matcher = void;
+
   // To implement a polymorphic matcher, we just need to make MatchAndExplain a
   // template on its first argument.