diff --git a/include/gsl.h b/include/gsl.h
index 745a078..373f2e8 100644
--- a/include/gsl.h
+++ b/include/gsl.h
@@ -196,6 +196,7 @@ private:
 };
 
 template<class T, class U> not_null<T> static_pointer_cast(not_null<U> const & ptr) { return not_null<T>{static_cast<T>(ptr.get())}; }
+template<class T, class U> not_null<T> const_pointer_cast(not_null<U> const & ptr) { return not_null<T>{const_cast<T>(ptr.get())}; }
 
 } // namespace gsl