diff --git a/.travis.yml b/.travis.yml
index d38b4db..d1ba971 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,6 +3,12 @@ language: php
 php:
   - 7.1
   - 7.2
+  - 7.3
+
+env:
+  matrix:
+    - COMPOSER_FLAGS="--prefer-lowest"
+    - COMPOSER_FLAGS=""
 
 before_script:
   - composer install --dev --prefer-source --no-interaction
diff --git a/tests/Unit/Model/FilterTest.php b/tests/Unit/Model/FilterTest.php
index 3e0929c..8de48fc 100644
--- a/tests/Unit/Model/FilterTest.php
+++ b/tests/Unit/Model/FilterTest.php
@@ -118,8 +118,8 @@ class FilterTest extends TestCase
     public function testMultipleSubjectsCanBeAdded()
     {
         $this->assertEquals(
-          ['subject' => '"Test"|"Foo bar"'],
-          $this->filter->subject(['Test', 'Foo bar'])->toArray()
+            ['subject' => '"Test"|"Foo bar"'],
+            $this->filter->subject(['Test', 'Foo bar'])->toArray()
         );
     }