diff --git a/include/gsl.h b/include/gsl.h
index bf3573e..1405993 100644
--- a/include/gsl.h
+++ b/include/gsl.h
@@ -178,6 +178,9 @@ class maybe_null_ret;
 template<class T>
 class maybe_null_dbg
 {
+    template<class U>
+    friend class maybe_null_dbg;
+
     static_assert(std::is_assignable<T&, std::nullptr_t>::value, "T cannot be assigned nullptr.");
 public:
     maybe_null_dbg() : ptr_(nullptr), tested_(false) {}
@@ -247,8 +250,10 @@ public:
 
     bool operator==(const T& rhs) const { tested_ = true; return ptr_ == rhs; }
     bool operator!=(const T& rhs) const { return !(*this == rhs); }
-    bool operator==(const maybe_null_dbg& rhs) const { tested_ = true; rhs.tested_ = true; return ptr_ == rhs.ptr_; }
-    bool operator!=(const maybe_null_dbg& rhs) const { return !(*this == rhs); }
+    template <typename U, typename Dummy = std::enable_if_t<std::is_convertible<U, T>::value>>
+    bool operator==(const maybe_null_dbg<U>& rhs) const { tested_ = true; rhs.tested_ = true; return ptr_ == rhs.ptr_; }
+    template <typename U, typename Dummy = std::enable_if_t<std::is_convertible<U, T>::value>>
+    bool operator!=(const maybe_null_dbg<U>& rhs) const { return !(*this == rhs); }
 
     T get() const {
         fail_fast_assert(tested_);
diff --git a/tests/maybenull_tests.cpp b/tests/maybenull_tests.cpp
index d6b53f0..944307e 100644
--- a/tests/maybenull_tests.cpp
+++ b/tests/maybenull_tests.cpp
@@ -178,13 +178,19 @@ SUITE(MaybeNullTests)
         maybe_null<MyBase*> q = p;
         CHECK(q == p);
 
+        maybe_null_dbg<MyDerived*> pdbg = &derived;
+        CHECK(pdbg.present());
+
+        maybe_null_dbg<MyBase*> qdbg = pdbg;
+        CHECK(qdbg == pdbg);
+
 #ifdef CONFIRM_COMPILATION_ERRORS
         maybe_null<Unrelated*> r = p;
         maybe_null<Unrelated*> s = reinterpret_cast<Unrelated*>(p);
 #endif
         maybe_null_dbg<Unrelated*> t = reinterpret_cast<Unrelated*>(p.get());
 
-	CHECK_THROW((void)(void*)t.get(), fail_fast);
+        CHECK_THROW((void)(void*)t.get(), fail_fast);
         maybe_null_dbg<Unrelated*> u = reinterpret_cast<Unrelated*>(p.get());
         CHECK(u.present());
         CHECK((void*)p.get() == (void*)u.get());