diff --git a/composer.json b/composer.json
index 4eff613..7d3295e 100644
--- a/composer.json
+++ b/composer.json
@@ -9,7 +9,8 @@
   },
   "require-dev": {
     "pestphp/pest": "^2.0",
-    "php-di/php-di": "^7.0"
+    "php-di/php-di": "^7.0",
+    "vimeo/psalm": "^5.13"
   },
   "license": "MIT",
   "autoload": {
diff --git a/psalm.xml b/psalm.xml
new file mode 100644
index 0000000..b86be40
--- /dev/null
+++ b/psalm.xml
@@ -0,0 +1,17 @@
+<?xml version="1.0"?>
+<psalm
+        errorLevel="7"
+        resolveFromConfigFile="true"
+        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xmlns="https://getpsalm.org/schema/config"
+        xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
+        findUnusedBaselineEntry="true"
+        findUnusedCode="false"
+>
+    <projectFiles>
+        <directory name="src"/>
+        <ignoreFiles>
+            <directory name="vendor"/>
+        </ignoreFiles>
+    </projectFiles>
+</psalm>
diff --git a/src/Container.php b/src/Container.php
index 01a20ea..eb85c21 100644
--- a/src/Container.php
+++ b/src/Container.php
@@ -79,8 +79,8 @@ public function delegateTo(ContainerInterface $container): void
     }
 
     /**
-     * @param  string  $class
-     * @return object|string|null
+     * @param string  $class
+     *
      * @throws ContainerException
      * @throws ContainerExceptionInterface
      * @throws NotFoundExceptionInterface
diff --git a/src/Definition.php b/src/Definition.php
index eadf465..70a6020 100644
--- a/src/Definition.php
+++ b/src/Definition.php
@@ -18,7 +18,7 @@ public function __construct(
     ) {
     }
 
-    public function singleton(): self
+    public function singleton(): static
     {
         $this->shared = true;
         return $this;
@@ -29,7 +29,7 @@ public function hasInstance(): bool
         return $this->instance !== null;
     }
 
-    public function getInstance(): object
+    public function getInstance(): object|null
     {
         return $this->instance;
     }