diff --git a/src/Monolog/DateTimeImmutable.php b/src/Monolog/DateTimeImmutable.php
index 968c58a6b..3cb7086d7 100644
--- a/src/Monolog/DateTimeImmutable.php
+++ b/src/Monolog/DateTimeImmutable.php
@@ -13,6 +13,7 @@
 
 class_alias(JsonSerializableDateTimeImmutable::class, 'Monolog\DateTimeImmutable');
 
+// @phpstan-ignore-next-line
 if (false) {
     /**
      * @deprecated Use \Monolog\JsonSerializableDateTimeImmutable instead.
diff --git a/src/Monolog/Formatter/GelfMessageFormatter.php b/src/Monolog/Formatter/GelfMessageFormatter.php
index 9b0861948..d169e1b61 100644
--- a/src/Monolog/Formatter/GelfMessageFormatter.php
+++ b/src/Monolog/Formatter/GelfMessageFormatter.php
@@ -115,6 +115,7 @@ public function format(LogRecord $record): Message
         }
 
         foreach ($extra as $key => $val) {
+            $key = (string) preg_replace('#[^\w\.\-]#', '-', $key);
             $val = \is_scalar($val) || null === $val ? $val : $this->toJson($val);
             $len = \strlen($this->extraPrefix . $key . $val);
             if ($len > $this->maxLength) {
@@ -126,6 +127,7 @@ public function format(LogRecord $record): Message
         }
 
         foreach ($context as $key => $val) {
+            $key = (string) preg_replace('#[^\w\.\-]#', '-', $key);
             $val = \is_scalar($val) || null === $val ? $val : $this->toJson($val);
             $len = \strlen($this->contextPrefix . $key . $val);
             if ($len > $this->maxLength) {