--- include/frozen/bits/defines.h
+++ include/frozen/bits/defines.h
@@ -59,7 +59,7 @@
   #define FROZEN_LETITGO_HAS_DEDUCTION_GUIDES
 #endif

-#if __cpp_lib_constexpr_string >= 201907L
+#if defined(__cpp_lib_constexpr_string) && __cpp_lib_constexpr_string >= 201907L
   #define FROZEN_LETITGO_HAS_CONSTEXPR_STRING
 #endif