diff --git a/include/boost/static_string/config.hpp b/include/boost/static_string/config.hpp
index fbec78a..1a1a49b 100644
--- a/include/boost/static_string/config.hpp
+++ b/include/boost/static_string/config.hpp
@@ -17,6 +17,9 @@
 // Disable exceptions and their associated checks
 // #define BOOST_STATIC_STRING_NO_EXCEPTIONS
 
+// Opt-in to the null terminator optimization
+// #define BOOST_STATIC_STRING_NULL_OPTIMIZATION
+
 // Can we have deduction guides?
 #ifdef __cpp_deduction_guides
 #define BOOST_STATIC_STRING_USE_DEDUCT
@@ -33,9 +36,6 @@
 #define BOOST_STATIC_STRING_NODISCARD
 #endif
 
-//#undef _­_­cplusplus
-//#define _­_­cplusplus 201703L
-
 #if __cplusplus > 201703L
 #define BOOST_STATIC_STRING_CPP20_CONSTEXPR constexpr
 #define BOOST_STATIC_STRING_CPP17_CONSTEXPR constexpr
@@ -114,6 +114,10 @@
 #endif
 #endif
 
+#if (__cplusplus >= 201402L) && (__cplusplus < 201703L) && defined(BOOST_STATIC_STRING_NULL_OPTIMIZATION) && defined(__clang__) && ((__clang_major__ == 4) || (__clang_major__ == 5))
+#error The null terminator optimization is not supported for clang 4.x and clang 5.x
+#endif
+
 namespace boost {
 namespace static_string {
 
diff --git a/include/boost/static_string/detail/static_string.hpp b/include/boost/static_string/detail/static_string.hpp
index 5032c40..d7fcf79 100644
--- a/include/boost/static_string/detail/static_string.hpp
+++ b/include/boost/static_string/detail/static_string.hpp
@@ -209,15 +209,13 @@ public:
 #endif
 };
 
-//#define BOOST_STATIC_STRING_NO_NULL_OPTIMIZATION
-
 // Decides which size optimization to use
 // If the size is zero, the object will have no members
 // Otherwise, if CharT can hold the max size of the string, store the size in the last char
 // Otherwise, store the size of the string using a member of the smallest type possible
 template<std::size_t N, typename CharT, typename Traits>
 using optimization_base = 
-#ifndef BOOST_STATIC_STRING_NO_NULL_OPTIMIZATION
+#ifdef BOOST_STATIC_STRING_USE_NULL_OPTIMIZATION
     typename std::conditional<(N <= (std::numeric_limits<CharT>::max)()) && (N != 0), 
         static_string_base_null<N, CharT, Traits>,
         static_string_base_zero<N, CharT, Traits>>::type;