diff --git a/src/Channel.php b/src/Channel.php
index c395a3f..9aadd92 100644
--- a/src/Channel.php
+++ b/src/Channel.php
@@ -382,7 +382,7 @@ public function txRollback(): \Bunny\Protocol\MethodTxRollbackOkFrame
     /**
      * Changes channel to confirm mode. Broker then asynchronously sends 'basic.ack's for published messages.
      */
-    public function confirmSelect(callable $callback = null, bool $nowait = false): \Bunny\Protocol\MethodConfirmSelectOkFrame
+    public function confirmSelect(?callable $callback = null, bool $nowait = false): \Bunny\Protocol\MethodConfirmSelectOkFrame
     {
         if ($this->mode !== ChannelMode::Regular) {
             throw new ChannelException("Channel not in regular mode, cannot change to transactional mode.");
@@ -394,7 +394,7 @@ public function confirmSelect(callable $callback = null, bool $nowait = false):
         return $response;
     }
 
-    private function enterConfirmMode(callable $callback = null): void
+    private function enterConfirmMode(?callable $callback = null): void
     {
         $this->mode = ChannelMode::Confirm;
         $this->deliveryTag = 0;
diff --git a/src/ChannelInterface.php b/src/ChannelInterface.php
index 4333a2e..312f0d3 100644
--- a/src/ChannelInterface.php
+++ b/src/ChannelInterface.php
@@ -114,6 +114,6 @@ public function txRollback(): \Bunny\Protocol\MethodTxRollbackOkFrame;
     /**
      * Changes channel to confirm mode. Broker then asynchronously sends 'basic.ack's for published messages.
      */
-    public function confirmSelect(callable $callback = null, bool $nowait = false): \Bunny\Protocol\MethodConfirmSelectOkFrame;
+    public function confirmSelect(?callable $callback = null, bool $nowait = false): \Bunny\Protocol\MethodConfirmSelectOkFrame;
 }
 
diff --git a/test/Library/SynchronousClientHelper.php b/test/Library/SynchronousClientHelper.php
index c6fc506..19e07dc 100644
--- a/test/Library/SynchronousClientHelper.php
+++ b/test/Library/SynchronousClientHelper.php
@@ -14,7 +14,7 @@ final class SynchronousClientHelper extends AbstractClientHelper
      *
      * @return Client
      */
-    public function createClient(array $options = null): Client
+    public function createClient(?array $options = null): Client
     {
         $options = array_merge($this->getDefaultOptions(), $options ?? []);