diff --git a/kafka/producer/kafka.py b/kafka/producer/kafka.py
index ae2877920..4a93de6d5 100644
--- a/kafka/producer/kafka.py
+++ b/kafka/producer/kafka.py
@@ -554,7 +554,7 @@ def send(self, topic, value=None, key=None, partition=None, timestamp_ms=None):
             partition = self._partition(topic, partition, key, value,
                                         key_bytes, value_bytes)
 
-            message_size = self._estimate_size_in_bytes(key, value)
+            message_size = self._estimate_size_in_bytes(key_bytes, value_bytes)
             self._ensure_valid_record_size(message_size)
 
             tp = TopicPartition(topic, partition)