diff --git a/src/Guid.php b/src/Guid.php
index 5503090..fdbf0aa 100644
--- a/src/Guid.php
+++ b/src/Guid.php
@@ -4,6 +4,7 @@
 
 namespace SmartEmailing\Types;
 
+use Nette\Utils\Strings;
 use SmartEmailing\Types\Comparable\ComparableInterface;
 use SmartEmailing\Types\Comparable\StringComparableTrait;
 use SmartEmailing\Types\ExtractableTraits\StringExtractableTrait;
@@ -15,12 +16,16 @@ final class Guid implements ToStringInterface, ComparableInterface
 	use ToStringTrait;
 	use StringComparableTrait;
 
+	private string $value;
+
 	private function __construct(
-		private string $value
+		string $value
 	) {
 		if (\preg_match('/^[a-f\d]{8}(-[a-f\d]{4}){4}[a-f\d]{8}$/i', $value) !== 1) {
 			throw new InvalidTypeException('Invalid guid value');
 		}
+
+		$this->value = Strings::lower($value);
 	}
 
 	public static function fromHex32(
diff --git a/tests/GuidTest.phpt b/tests/GuidTest.phpt
index 8384ef4..0c9a5d5 100644
--- a/tests/GuidTest.phpt
+++ b/tests/GuidTest.phpt
@@ -4,6 +4,7 @@ declare(strict_types = 1);
 
 namespace SmartEmailing\Types;
 
+use Nette\Utils\Strings;
 use Tester\Assert;
 use Tester\TestCase;
 
@@ -32,11 +33,16 @@ final class GuidTest extends TestCase
 
 		$validValues = [
 			'd7c8539e-089e-11e8-b161-2edbc134be21',
+			'd7c8539e-089E-11E8-B161-2edbc134be21',
 		];
 
 		foreach ($validValues as $validValue) {
 			$guid = Guid::from($validValue);
-			Assert::equal($validValue, $guid->getValue());
+
+			Assert::equal(
+				Strings::lower($validValue),
+				$guid->getValue()
+			);
 		}
 	}