diff --git a/lib/fluent/logger/fluent_logger.rb b/lib/fluent/logger/fluent_logger.rb
index 3d8578f..7cb108b 100644
--- a/lib/fluent/logger/fluent_logger.rb
+++ b/lib/fluent/logger/fluent_logger.rb
@@ -177,7 +177,7 @@ def connection_string
 
       def pending_bytesize
         if @pending
-          @pending.bytesize
+          @pending.to_s.bytesize
         else
           0
         end