diff --git a/include/nlohmann/detail/output/binary_writer.hpp b/include/nlohmann/detail/output/binary_writer.hpp
index a9291bcbd..dbe9484b9 100644
--- a/include/nlohmann/detail/output/binary_writer.hpp
+++ b/include/nlohmann/detail/output/binary_writer.hpp
@@ -1372,7 +1372,8 @@ 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)
         {
@@ -1459,7 +1460,8 @@ 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 b94841147..ac22b51a0 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -20334,7 +20334,8 @@ 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)
         {
@@ -20421,7 +20422,8 @@ 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)
         {