diff --git a/src/MemorySession.php b/src/MemorySession.php
index 3eafa4b..7f090a9 100644
--- a/src/MemorySession.php
+++ b/src/MemorySession.php
@@ -77,7 +77,7 @@ public function getName(): string
         return $this->options['name'];
     }
 
-    public function get(string $key, $default = null)
+    public function get(string $key, mixed $default = null): mixed
     {
         return $this->storage[$key] ?? $default;
     }
@@ -87,7 +87,7 @@ public function all(): array
         return (array)$this->storage;
     }
 
-    public function set(string $key, $value): void
+    public function set(string $key, mixed $value): void
     {
         $this->storage[$key] = $value;
     }
diff --git a/src/PhpSession.php b/src/PhpSession.php
index 8bfdba6..e882e03 100644
--- a/src/PhpSession.php
+++ b/src/PhpSession.php
@@ -149,7 +149,7 @@ public function getName(): string
         return (string)session_name();
     }
 
-    public function get(string $key, $default = null)
+    public function get(string $key, mixed $default = null): mixed
     {
         return $this->storage[$key] ?? $default;
     }
@@ -159,7 +159,7 @@ public function all(): array
         return (array)$this->storage;
     }
 
-    public function set(string $key, $value): void
+    public function set(string $key, mixed $value): void
     {
         $this->storage[$key] = $value;
     }
diff --git a/src/SessionInterface.php b/src/SessionInterface.php
index 2f63579..fc76307 100644
--- a/src/SessionInterface.php
+++ b/src/SessionInterface.php
@@ -3,7 +3,7 @@
 namespace Odan\Session;
 
 /**
- * Interface.
+ * The session data operations.
  */
 interface SessionInterface
 {
@@ -11,11 +11,11 @@ interface SessionInterface
      * Gets an attribute by key.
      *
      * @param string $key The key name or null to get all values
-     * @param null $default The default value
+     * @param mixed $default The default value
      *
-     * @return mixed|null Should return null if the key is not found
+     * @return mixed The value. Returns null if the key is not found
      */
-    public function get(string $key, $default = null);
+    public function get(string $key, mixed $default = null): mixed;
 
     /**
      * Gets all values as array.
@@ -32,7 +32,7 @@ public function all(): array;
      *
      * @return void
      */
-    public function set(string $key, $value): void;
+    public function set(string $key, mixed $value): void;
 
     /**
      * Sets multiple attributes at once: takes a keyed array and sets each key => value pair.