diff --git a/src/stan/callbacks/unique_stream_writer.hpp b/src/stan/callbacks/unique_stream_writer.hpp
index f763b23650..389d96ef0e 100644
--- a/src/stan/callbacks/unique_stream_writer.hpp
+++ b/src/stan/callbacks/unique_stream_writer.hpp
@@ -113,6 +113,7 @@ class unique_stream_writer final : public writer {
       return;
     *output_ << comment_prefix_ << message << std::endl;
   }
+
  private:
   /**
    * Comma formatter for writing Eigen matrices