diff --git a/src/Enum.php b/src/Enum.php
index cfb4bd8..ca6aef0 100644
--- a/src/Enum.php
+++ b/src/Enum.php
@@ -28,6 +28,13 @@ abstract class Enum implements \JsonSerializable
      */
     protected $value;
 
+    /**
+     * Enum key, the constant name
+     *
+     * @var string
+     */
+    private $key;
+
     /**
      * Store existing constants in a static cache per object.
      *
@@ -61,7 +68,7 @@ public function __construct($value)
             $value = $value->getValue();
         }
 
-        static::assertValidValue($value);
+        $this->key = static::assertValidValue($value);
 
         /** @psalm-var T */
         $this->value = $value;
@@ -94,9 +101,9 @@ public function getValue()
      *
      * @psalm-pure
      */
-    public function getKey(): string
+    public function getKey()
     {
-        return static::search($this->value);
+        return $this->key ?? ($this->key = static::search($this->value));
     }
 
     /**
@@ -198,11 +205,13 @@ public static function isValid($value)
      * @psalm-pure
      * @psalm-assert T $value
      */
-    private static function assertValidValue($value): void
+    private static function assertValidValue($value): string
     {
-        if (!static::isValid($value)) {
+        if (false === ($key = static::search($value))) {
             throw new \UnexpectedValueException("Value '$value' is not part of the enum " . static::class);
         }
+
+        return $key;
     }
 
     /**
diff --git a/tests/EnumTest.php b/tests/EnumTest.php
index e8dce2e..f1d18ac 100755
--- a/tests/EnumTest.php
+++ b/tests/EnumTest.php
@@ -324,7 +324,8 @@ public function testSerialize()
     {
         // split string for Pretty CI: "Line exceeds 120 characters"
         $bin = '4f3a33303a224d79434c6162735c54657374735c456e756d5c456e756d4669787'.
-            '4757265223a313a7b733a383a22002a0076616c7565223b733a333a22666f6f223b7d';
+            '4757265223a323a7b733a383a22002a0076616c7565223b733a333a22666f6f223b73'.
+            '3a32323a22004d79434c6162735c456e756d5c456e756d006b6579223b733a333a22464f4f223b7d';
 
         $this->assertEquals($bin, bin2hex(serialize(EnumFixture::FOO())));
     }