diff --git a/gsl/gsl b/gsl/gsl
index 12a9676..8d67933 100644
--- a/gsl/gsl
+++ b/gsl/gsl
@@ -78,7 +78,7 @@ class not_null
     static_assert(std::is_assignable<T&, std::nullptr_t>::value, "T cannot be assigned nullptr.");
 
 public:
-    not_null(T t) : ptr_(t) { ensure_invariant(); }
+    explicit not_null(T t) : ptr_(t) { ensure_invariant(); }
     not_null& operator=(const T& t)
     {
         ptr_ = t;