diff --git a/src/FileMessage.php b/src/FileMessage.php
index bd39ef8..49577b9 100644
--- a/src/FileMessage.php
+++ b/src/FileMessage.php
@@ -2,6 +2,7 @@
 
 namespace DirectoryTree\ImapEngine;
 
+use BackedEnum;
 use BadMethodCallException;
 
 class FileMessage implements MessageInterface
@@ -26,7 +27,7 @@ public function uid(): int
     /**
      * {@inheritDoc}
      */
-    public function flag(mixed $flag, string $operation, bool $expunge = false): void
+    public function flag(BackedEnum|string $flag, string $operation, bool $expunge = false): void
     {
         throw new BadMethodCallException('FileMessage does not support flagging');
     }
diff --git a/src/FlaggableInterface.php b/src/FlaggableInterface.php
index 62a0d3c..3051352 100644
--- a/src/FlaggableInterface.php
+++ b/src/FlaggableInterface.php
@@ -121,5 +121,5 @@ public function hasFlag(BackedEnum|string $flag): bool;
     /**
      * Add or remove a flag from the message.
      */
-    public function flag(mixed $flag, string $operation, bool $expunge = false): void;
+    public function flag(BackedEnum|string $flag, string $operation, bool $expunge = false): void;
 }
diff --git a/src/HasFlags.php b/src/HasFlags.php
index d6e3ee8..791c460 100644
--- a/src/HasFlags.php
+++ b/src/HasFlags.php
@@ -184,5 +184,5 @@ abstract public function flags(): array;
     /**
      * {@inheritDoc}
      */
-    abstract public function flag(mixed $flag, string $operation, bool $expunge = false): void;
+    abstract public function flag(BackedEnum|string $flag, string $operation, bool $expunge = false): void;
 }
diff --git a/src/Message.php b/src/Message.php
index 10f3b62..492d6bd 100644
--- a/src/Message.php
+++ b/src/Message.php
@@ -2,6 +2,7 @@
 
 namespace DirectoryTree\ImapEngine;
 
+use BackedEnum;
 use DirectoryTree\ImapEngine\Connection\Responses\MessageResponseParser;
 use DirectoryTree\ImapEngine\Exceptions\ImapCapabilityException;
 use DirectoryTree\ImapEngine\Support\Str;
@@ -103,7 +104,7 @@ public function is(MessageInterface $message): bool
     /**
      * Add or remove a flag from the message.
      */
-    public function flag(mixed $flag, string $operation, bool $expunge = false): void
+    public function flag(BackedEnum|string $flag, string $operation, bool $expunge = false): void
     {
         $flag = Str::enum($flag);
 
diff --git a/src/Testing/FakeMessage.php b/src/Testing/FakeMessage.php
index a08e447..2a93be2 100644
--- a/src/Testing/FakeMessage.php
+++ b/src/Testing/FakeMessage.php
@@ -2,6 +2,7 @@
 
 namespace DirectoryTree\ImapEngine\Testing;
 
+use BackedEnum;
 use DirectoryTree\ImapEngine\HasFlags;
 use DirectoryTree\ImapEngine\HasParsedMessage;
 use DirectoryTree\ImapEngine\MessageInterface;
@@ -42,7 +43,7 @@ public function is(MessageInterface $message): bool
     /**
      * {@inheritDoc}
      */
-    public function flag(mixed $flag, string $operation, bool $expunge = false): void
+    public function flag(BackedEnum|string $flag, string $operation, bool $expunge = false): void
     {
         $flag = Str::enum($flag);