diff --git a/lib/fluent/plugin/output.rb b/lib/fluent/plugin/output.rb
index f2e3987012..3d64839264 100644
--- a/lib/fluent/plugin/output.rb
+++ b/lib/fluent/plugin/output.rb
@@ -874,7 +874,7 @@ def write_guard(&block)
             begin
               oldest = @buffer.dequeue_chunk
               if oldest
-                log.warn "dropping oldest chunk to make space after buffer overflow", chunk_id: oldest.unique_id
+                log.warn "dropping oldest chunk to make space after buffer overflow", chunk_id: dump_unique_id_hex(oldest.unique_id)
                 @buffer.purge_chunk(oldest.unique_id)
               else
                 log.error "no queued chunks to be dropped for drop_oldest_chunk"