diff --git a/include/gsl/pointers b/include/gsl/pointers
index b2804a3..ad15ce3 100644
--- a/include/gsl/pointers
+++ b/include/gsl/pointers
@@ -72,7 +72,7 @@ public:
     static_assert(std::is_assignable<T&, std::nullptr_t>::value, "T cannot be assigned nullptr.");
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr not_null(U&& u) : ptr_(std::forward<U>(u))
+    constexpr explicit not_null(U&& u) : ptr_(std::forward<U>(u))
     {
         Expects(ptr_ != nullptr);
     }
diff --git a/tests/notnull_tests.cpp b/tests/notnull_tests.cpp
index a7ef27e..fa99bb7 100644
--- a/tests/notnull_tests.cpp
+++ b/tests/notnull_tests.cpp
@@ -178,10 +178,10 @@ TEST_CASE("TestNotNullCasting")
     MyBase base;
     MyDerived derived;
     Unrelated unrelated;
-    not_null<Unrelated*> u = &unrelated;
+    not_null<Unrelated*> u{&unrelated};
     (void) u;
-    not_null<MyDerived*> p = &derived;
-    not_null<MyBase*> q = &base;
+    not_null<MyDerived*> p{&derived};
+    not_null<MyBase*> q(&base);
     q = p; // allowed with heterogeneous copy ctor
     CHECK(q == p);
 
@@ -192,18 +192,18 @@ TEST_CASE("TestNotNullCasting")
     not_null<Unrelated*> r = p;
     not_null<Unrelated*> s = reinterpret_cast<Unrelated*>(p);
 #endif
-    not_null<Unrelated*> t = reinterpret_cast<Unrelated*>(p.get());
+    not_null<Unrelated*> t(reinterpret_cast<Unrelated*>(p.get()));
     CHECK(reinterpret_cast<void*>(p.get()) == reinterpret_cast<void*>(t.get()));
 }
 
 TEST_CASE("TestNotNullAssignment")
 {
     int i = 12;
-    not_null<int*> p = &i;
+    not_null<int*> p(&i);
     CHECK(helper(p));
 
     int* q = nullptr;
-    CHECK_THROWS_AS(p = q, fail_fast);
+    CHECK_THROWS_AS(p = not_null<int*>(q), fail_fast);
 }
 
 TEST_CASE("TestNotNullRawPointerComparison")