diff --git a/libcxx/include/string b/libcxx/include/string
index ede42467b99fe..ab44f25bafa0e 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -918,7 +918,14 @@ private:
 
     __rep() = default;
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __rep(__short __r) : __s(__r) {}
-    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __rep(__long __r) : __l(__r) {}
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __rep(__long __r) : __s() {
+      // For nonstandard value_type, __s can be bigger than __l, because it
+      // contains at least two value_type. So if we're storing a long buffer,
+      // we must set the is_long flag in __s, where __is_long() will look, in
+      // case it isn't set by assigning to __l.
+      __s.__is_long_ = true;
+      __l            = __r;
+    }
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __rep(__uninitialized_tag) {}
   };
 
diff --git a/libcxx/test/std/strings/basic.string/string.modifiers/string_append/push_back.pass.cpp b/libcxx/test/std/strings/basic.string/string.modifiers/string_append/push_back.pass.cpp
index 8c2324c9d1759..34669c28d624c 100644
--- a/libcxx/test/std/strings/basic.string/string.modifiers/string_append/push_back.pass.cpp
+++ b/libcxx/test/std/strings/basic.string/string.modifiers/string_append/push_back.pass.cpp
@@ -84,9 +84,13 @@ TEST_CONSTEXPR_CXX20 bool test() {
     // https://llvm.org/PR31454
     std::basic_string<VeryLarge> s;
     VeryLarge vl = {};
+    LIBCPP_ASSERT(s.size() == 0);
     s.push_back(vl);
+    LIBCPP_ASSERT(s.size() == 1);
     s.push_back(vl);
+    LIBCPP_ASSERT(s.size() == 2);
     s.push_back(vl);
+    LIBCPP_ASSERT(s.size() == 3);
   }
 
   return true;