diff --git a/include/experimental/__p0009_bits/macros.hpp b/include/experimental/__p0009_bits/macros.hpp
index c15cfcbc..9379c0c9 100644
--- a/include/experimental/__p0009_bits/macros.hpp
+++ b/include/experimental/__p0009_bits/macros.hpp
@@ -152,7 +152,7 @@ MDSPAN_FUNCTION inline void default_precondition_violation_handler(const char* c
 #endif
 
 #ifndef MDSPAN_IMPL_CHECK_PRECONDITION
-  #ifndef NDEBUG
+  #ifdef NDEBUG
     #define MDSPAN_IMPL_CHECK_PRECONDITION 0
   #else
     #define MDSPAN_IMPL_CHECK_PRECONDITION 1