diff --git a/include/nlohmann/detail/output/binary_writer.hpp b/include/nlohmann/detail/output/binary_writer.hpp
index 24e7c1094..7cd3d804b 100644
--- a/include/nlohmann/detail/output/binary_writer.hpp
+++ b/include/nlohmann/detail/output/binary_writer.hpp
@@ -908,6 +908,7 @@ class binary_writer
         if (JSON_HEDLEY_UNLIKELY(it != BasicJsonType::string_t::npos))
         {
             JSON_THROW(out_of_range::create(409, "BSON key cannot contain code point U+0000 (at byte " + std::to_string(it) + ")", j));
+            static_cast<void>(j);
         }
 
         return /*id*/ 1ul + name.size() + /*zero-terminator*/1u;
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 28ce55292..681f19495 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -14209,6 +14209,7 @@ class binary_writer
         if (JSON_HEDLEY_UNLIKELY(it != BasicJsonType::string_t::npos))
         {
             JSON_THROW(out_of_range::create(409, "BSON key cannot contain code point U+0000 (at byte " + std::to_string(it) + ")", j));
+            static_cast<void>(j);
         }
 
         return /*id*/ 1ul + name.size() + /*zero-terminator*/1u;