diff --git a/src/Driver/AmqpExtension/Envelope.php b/src/Driver/AmqpExtension/Envelope.php
index 702748a..40ca8a3 100644
--- a/src/Driver/AmqpExtension/Envelope.php
+++ b/src/Driver/AmqpExtension/Envelope.php
@@ -128,7 +128,7 @@ public function getHeaders(): array
         return $this->envelope->getHeaders();
     }
 
-    public function getHeader(string $header): ?string
+    public function getHeader(string $header)
     {
         if (! $this->hasHeader($header)) {
             return null;
diff --git a/src/Driver/PhpAmqpLib/Envelope.php b/src/Driver/PhpAmqpLib/Envelope.php
index 6337f03..068378d 100644
--- a/src/Driver/PhpAmqpLib/Envelope.php
+++ b/src/Driver/PhpAmqpLib/Envelope.php
@@ -147,7 +147,7 @@ public function getHeaders(): array
         return [];
     }
 
-    public function getHeader(string $header): ?string
+    public function getHeader(string $header)
     {
         $headers = $this->getHeaders();
 
diff --git a/src/Envelope.php b/src/Envelope.php
index 593c0bd..50b13b1 100644
--- a/src/Envelope.php
+++ b/src/Envelope.php
@@ -165,9 +165,9 @@ public function getHeaders(): array;
      * Get a specific message header.
      *
      * @param string $header Name of the header to get the value from.
-     * @return string|null The contents of the specified header, null if header not set
+     * @return mixed The contents of the specified header, null if header not set
      */
-    public function getHeader(string $header): ?string;
+    public function getHeader(string $header);
 
     /**
      * Check whether a specific message header is set.