diff --git a/include/nlohmann/detail/input/binary_reader.hpp b/include/nlohmann/detail/input/binary_reader.hpp
index 5093e032b..205db484b 100644
--- a/include/nlohmann/detail/input/binary_reader.hpp
+++ b/include/nlohmann/detail/input/binary_reader.hpp
@@ -2970,6 +2970,9 @@ class binary_reader
                                        : chunk_size;
             const std::size_t old_size = result.size();
             result.resize(old_size + wanted);
+            // the destination now has room for `wanted` bytes at offset
+            // old_size, which is what get_elements() is allowed to write
+            JSON_ASSERT(result.size() >= old_size + wanted);
             const std::size_t bytes_read = ia.get_elements(&result[old_size], wanted);
             chars_read += bytes_read;
             if (JSON_HEDLEY_UNLIKELY(bytes_read < wanted))
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index bcc124f14..dea47a62d 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -13176,6 +13176,9 @@ class binary_reader
                                        : chunk_size;
             const std::size_t old_size = result.size();
             result.resize(old_size + wanted);
+            // the destination now has room for `wanted` bytes at offset
+            // old_size, which is what get_elements() is allowed to write
+            JSON_ASSERT(result.size() >= old_size + wanted);
             const std::size_t bytes_read = ia.get_elements(&result[old_size], wanted);
             chars_read += bytes_read;
             if (JSON_HEDLEY_UNLIKELY(bytes_read < wanted))