diff --git a/src/Elasticsearch/Serializers/SmartSerializer.php b/src/Elasticsearch/Serializers/SmartSerializer.php
index 6fa200f39..8317a1743 100644
--- a/src/Elasticsearch/Serializers/SmartSerializer.php
+++ b/src/Elasticsearch/Serializers/SmartSerializer.php
@@ -33,7 +33,7 @@ public function serialize($data): string
         } else {
             $data = json_encode($data, JSON_PRESERVE_ZERO_FRACTION + JSON_INVALID_UTF8_SUBSTITUTE);
             if ($data === false) {
-                throw new Exceptions\RuntimeException("Failed to JSON encode: ".json_last_error());
+                throw new Exceptions\RuntimeException("Failed to JSON encode: ".json_last_error_msg());
             }
             if ($data === '[]') {
                 return '{}';