diff --git a/src/Message/AbstractEnvelope.php b/src/Message/AbstractEnvelope.php
index f8482e1c..66b44e54 100644
--- a/src/Message/AbstractEnvelope.php
+++ b/src/Message/AbstractEnvelope.php
@@ -24,7 +24,7 @@ public function __construct(MessageInterface $message)
         }
         $this->message = $message;
 
-        if (is_array($this->metadata[EnvelopeInterface::ENVELOPE_STACK_KEY])) {
+        if (isset($this->metadata[EnvelopeInterface::ENVELOPE_STACK_KEY]) && is_array($this->metadata[EnvelopeInterface::ENVELOPE_STACK_KEY])) {
             $this->metadata[EnvelopeInterface::ENVELOPE_STACK_KEY] = array_merge(
                 $envelopes,
                 array_filter(
diff --git a/src/Middleware/FailureHandling/FailureEnvelope.php b/src/Middleware/FailureHandling/FailureEnvelope.php
index 4ccaf0f4..97e194d6 100644
--- a/src/Middleware/FailureHandling/FailureEnvelope.php
+++ b/src/Middleware/FailureHandling/FailureEnvelope.php
@@ -26,6 +26,6 @@ public static function fromMessage(MessageInterface $message): self
 
     protected function getEnvelopeMetadata(): array
     {
-        return [self::FAILURE_META_KEY => ArrayHelper::merge($this->metadata[self::FAILURE_META_KEY], $this->failureMeta)];
+        return [self::FAILURE_META_KEY => ArrayHelper::merge($this->metadata[self::FAILURE_META_KEY] ?? [], $this->failureMeta)];
     }
 }