diff --git a/include/nlohmann/detail/output/binary_writer.hpp b/include/nlohmann/detail/output/binary_writer.hpp
index 89b8ce663..ef26732b9 100644
--- a/include/nlohmann/detail/output/binary_writer.hpp
+++ b/include/nlohmann/detail/output/binary_writer.hpp
@@ -1368,7 +1368,8 @@ class binary_writer
         // is in; nothing is allocated unless the document nests
         bson_frame current(&document);
         std::vector<bson_frame> parents;
-        string_t index_name;
+        // string_t need not be default constructible
+        string_t index_name("", 0);
 
         while (true)
         {
@@ -1455,7 +1456,8 @@ class binary_writer
         bson_frame current(&document);
         std::vector<bson_frame> parents;
         std::size_t next_size = 0;
-        string_t index_name;
+        // string_t need not be default constructible
+        string_t index_name("", 0);
 
         while (true)
         {
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 9716832c1..9d059a01e 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -20396,7 +20396,8 @@ class binary_writer
         // is in; nothing is allocated unless the document nests
         bson_frame current(&document);
         std::vector<bson_frame> parents;
-        string_t index_name;
+        // string_t need not be default constructible
+        string_t index_name("", 0);
 
         while (true)
         {
@@ -20483,7 +20484,8 @@ class binary_writer
         bson_frame current(&document);
         std::vector<bson_frame> parents;
         std::size_t next_size = 0;
-        string_t index_name;
+        // string_t need not be default constructible
+        string_t index_name("", 0);
 
         while (true)
         {