diff --git a/folly/FixedString.h b/folly/FixedString.h
index 54a7268c06c..448d626400e 100644
--- a/folly/FixedString.h
+++ b/folly/FixedString.h
@@ -58,8 +58,10 @@ struct FixedStringBase_ {
   static constexpr std::size_t npos = static_cast<std::size_t>(-1);
 };
 
+#if FOLLY_CPLUSPLUS < 201703L
 template <class Void>
 constexpr std::size_t FixedStringBase_<Void>::npos;
+#endif
 
 using FixedStringBase = FixedStringBase_<>;