diff --git a/src/Monolog/Utils.php b/src/Monolog/Utils.php
index e2058d59..240b3742 100644
--- a/src/Monolog/Utils.php
+++ b/src/Monolog/Utils.php
@@ -13,7 +13,7 @@ namespace Monolog;
 
 final class Utils
 {
-    const DEFAULT_JSON_FLAGS = JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE | JSON_PRESERVE_ZERO_FRACTION;
+    const DEFAULT_JSON_FLAGS = JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE | JSON_PRESERVE_ZERO_FRACTION | JSON_INVALID_UTF8_SUBSTITUTE;
 
     /**
      * @internal