diff --git a/module/Rest/src/Action/CreateShortcodeAction.php b/module/Rest/src/Action/CreateShortcodeAction.php
index a4f7919b..790c361b 100644
--- a/module/Rest/src/Action/CreateShortcodeAction.php
+++ b/module/Rest/src/Action/CreateShortcodeAction.php
@@ -74,7 +74,7 @@ class CreateShortcodeAction extends AbstractRestAction
 
             return new JsonResponse([
                 'longUrl' => $longUrl,
-                'shortUrl' => $shortUrl->__toString(),
+                'shortUrl' => (string) $shortUrl,
                 'shortCode' => $shortCode,
             ]);
         } catch (InvalidUrlException $e) {
diff --git a/module/Rest/test/Action/CreateShortcodeActionTest.php b/module/Rest/test/Action/CreateShortcodeActionTest.php
index adb1ada4..ce297105 100644
--- a/module/Rest/test/Action/CreateShortcodeActionTest.php
+++ b/module/Rest/test/Action/CreateShortcodeActionTest.php
@@ -7,6 +7,7 @@ use PHPUnit\Framework\TestCase;
 use Prophecy\Argument;
 use Prophecy\Prophecy\ObjectProphecy;
 use Shlinkio\Shlink\Core\Exception\InvalidUrlException;
+use Shlinkio\Shlink\Core\Exception\NonUniqueSlugException;
 use Shlinkio\Shlink\Core\Service\UrlShortener;
 use Shlinkio\Shlink\Rest\Action\CreateShortcodeAction;
 use Shlinkio\Shlink\Rest\Util\RestUtils;
@@ -52,7 +53,7 @@ class CreateShortcodeActionTest extends TestCase
      */
     public function properShortcodeConversionReturnsData()
     {
-        $this->urlShortener->urlToShortCode(Argument::type(Uri::class), Argument::type('array'), null, null)
+        $this->urlShortener->urlToShortCode(Argument::type(Uri::class), Argument::type('array'), Argument::cetera())
             ->willReturn('abc123')
             ->shouldBeCalledTimes(1);
 
@@ -69,7 +70,7 @@ class CreateShortcodeActionTest extends TestCase
      */
     public function anInvalidUrlReturnsError()
     {
-        $this->urlShortener->urlToShortCode(Argument::type(Uri::class), Argument::type('array'), null, null)
+        $this->urlShortener->urlToShortCode(Argument::type(Uri::class), Argument::type('array'), Argument::cetera())
             ->willThrow(InvalidUrlException::class)
             ->shouldBeCalledTimes(1);
 
@@ -81,12 +82,30 @@ class CreateShortcodeActionTest extends TestCase
         $this->assertTrue(strpos($response->getBody()->getContents(), RestUtils::INVALID_URL_ERROR) > 0);
     }
 
+    /**
+     * @test
+     */
+    public function nonUniqueSlugReturnsError()
+    {
+        $this->urlShortener->urlToShortCode(Argument::type(Uri::class), Argument::type('array'), null, null, 'foo')
+            ->willThrow(NonUniqueSlugException::class)
+            ->shouldBeCalledTimes(1);
+
+        $request = ServerRequestFactory::fromGlobals()->withParsedBody([
+            'longUrl' => 'http://www.domain.com/foo/bar',
+            'customSlug' => 'foo',
+        ]);
+        $response = $this->action->process($request, TestUtils::createDelegateMock()->reveal());
+        $this->assertEquals(400, $response->getStatusCode());
+        $this->assertContains(RestUtils::INVALID_SLUG_ERROR, (string) $response->getBody());
+    }
+
     /**
      * @test
      */
     public function aGenericExceptionWillReturnError()
     {
-        $this->urlShortener->urlToShortCode(Argument::type(Uri::class), Argument::type('array'), null, null)
+        $this->urlShortener->urlToShortCode(Argument::type(Uri::class), Argument::type('array'), Argument::cetera())
             ->willThrow(\Exception::class)
             ->shouldBeCalledTimes(1);