diff --git a/include/boost/static_string/config.hpp b/include/boost/static_string/config.hpp
index 1a1a49b..2a7af47 100644
--- a/include/boost/static_string/config.hpp
+++ b/include/boost/static_string/config.hpp
@@ -114,8 +114,13 @@
 #endif
 #endif
 
-#if (__cplusplus >= 201402L) && (__cplusplus < 201703L) && defined(BOOST_STATIC_STRING_NULL_OPTIMIZATION) && defined(__clang__) && ((__clang_major__ == 4) || (__clang_major__ == 5))
+#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
+#undef BOOST_STATIC_STRING_NULL_OPTIMIZATION
 #endif
 
 namespace boost {
diff --git a/test/static_string.cpp b/test/static_string.cpp
index f8d7c4b..9c3f8c3 100644
--- a/test/static_string.cpp
+++ b/test/static_string.cpp
@@ -6974,7 +6974,7 @@ constexpr bool testConstexpr()
   //constexpr auto h = s1.assign({'1'});
   //constexpr auto i = s1.assign("", 1);
   
-#else __cplusplus >= 201103L
+#elif __cplusplus >= 201103L
   // c++11 constexpr tests
   /*basic_static_string<4, char, cxper_char_traits> a;
   auto b = a.size();