diff --git a/include/nlohmann/detail/output/binary_writer.hpp b/include/nlohmann/detail/output/binary_writer.hpp
index 4b481e61d..6e5592839 100644
--- a/include/nlohmann/detail/output/binary_writer.hpp
+++ b/include/nlohmann/detail/output/binary_writer.hpp
@@ -1358,6 +1358,14 @@ class binary_writer
             oa.write_character(to_char_type(0x00));
             if (parents.empty())
             {
+                // calc_bson_sizes() and write_bson_document() are two
+                // hand-synchronized passes over the same structure, linked
+                // only by nested_sizes' visiting order; this checks that the
+                // write pass consumed exactly the sizes the size pass
+                // produced, so a future change that desyncs them (skips or
+                // rejects an entry in only one pass) is caught immediately
+                // instead of silently writing wrong length prefixes.
+                JSON_ASSERT(next_size == nested_sizes.size());
                 return;
             }
             current = std::move(parents.back());
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index cc1a08824..8024c58b5 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -21651,6 +21651,14 @@ class binary_writer
             oa.write_character(to_char_type(0x00));
             if (parents.empty())
             {
+                // calc_bson_sizes() and write_bson_document() are two
+                // hand-synchronized passes over the same structure, linked
+                // only by nested_sizes' visiting order; this checks that the
+                // write pass consumed exactly the sizes the size pass
+                // produced, so a future change that desyncs them (skips or
+                // rejects an entry in only one pass) is caught immediately
+                // instead of silently writing wrong length prefixes.
+                JSON_ASSERT(next_size == nested_sizes.size());
                 return;
             }
             current = std::move(parents.back());