diff --git a/src/Monolog/Formatter/LineFormatter.php b/src/Monolog/Formatter/LineFormatter.php
index a9e4a415..cfffb267 100644
--- a/src/Monolog/Formatter/LineFormatter.php
+++ b/src/Monolog/Formatter/LineFormatter.php
@@ -146,7 +146,7 @@ class LineFormatter extends NormalizerFormatter
             return (string) $data;
         }
 
-        return $this->toJson($data, true);
+        return (string) $this->toJson($data, true);
     }
 
     protected function replaceNewlines(string $str): string