diff --git a/.travis.yml b/.travis.yml
index d3d1891..5fc86c6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,9 +5,7 @@ matrix:
         - php: 7.1
           env:
             - COMPOSER_FLAGS='--prefer-lowest'
-        - php: 7.2
-
-sudo: false
+        - php: 7.3
 
 cache:
     directories:
diff --git a/Form/Type/ResourceType.php b/Form/Type/ResourceType.php
index 352919c..a01cfd4 100644
--- a/Form/Type/ResourceType.php
+++ b/Form/Type/ResourceType.php
@@ -17,17 +17,17 @@
 use Symfony\Component\Form\FormBuilderInterface;
 use Symfony\Component\OptionsResolver\OptionsResolver;
 
-class ResourceType extends AbstractType
+final class ResourceType extends AbstractType
 {
     /**
      * @var MapBuilder
      */
-    protected $mapBuilder;
+    private $mapBuilder;
 
     /**
      * @var string
      */
-    protected $resourceClass;
+    private $resourceClass;
 
     public function __construct(MapBuilder $mapBuilder, string $resourceClass)
     {
@@ -49,7 +49,7 @@ public function configureOptions(OptionsResolver $resolver)
     public function buildForm(FormBuilderInterface $builder, array $options)
     {
         if (false === $this->mapBuilder->hasResource($options['resource_key'])) {
-            throw new ResourceFormTypeException(sprintf('"%s" is not a valid resource key', $options['resource_key']));
+            throw new ResourceFormTypeException("\"{$options['resource_key']}\" is not a valid resource key");
         }
 
         $resource = $this->mapBuilder->getResource($options['resource_key']);
diff --git a/Model/Resource.php b/Model/Resource.php
index 9975d7e..5af9d4c 100644
--- a/Model/Resource.php
+++ b/Model/Resource.php
@@ -11,7 +11,9 @@
 
 namespace FSi\Bundle\ResourceRepositoryBundle\Model;
 
+use DateTime;
 use DateTimeImmutable;
+use DateTimeInterface;
 
 class Resource implements ResourceValue
 {
@@ -82,7 +84,7 @@ public function getTextValue()
 
     public function setDateValue($dateValue)
     {
-        $this->dateValue = $dateValue;
+        $this->dateValue = $this->toDateTimeImmutable($dateValue);
     }
 
     public function getDateValue()
@@ -92,7 +94,7 @@ public function getDateValue()
 
     public function setDatetimeValue($datetimeValue)
     {
-        $this->datetimeValue = $datetimeValue;
+        $this->datetimeValue = $this->toDateTimeImmutable($datetimeValue);
     }
 
     public function getDatetimeValue()
@@ -102,7 +104,7 @@ public function getDatetimeValue()
 
     public function setTimeValue($timeValue)
     {
-        $this->timeValue = $timeValue;
+        $this->timeValue = $this->toDateTimeImmutable($timeValue);
     }
 
     public function getTimeValue()
@@ -139,4 +141,20 @@ public function getBoolValue()
     {
         return $this->boolValue;
     }
+
+    /**
+     * Symfony date/time/datetime forms do not allow for default DateTimeImmutable
+     * value until version 4.2, so the values need to be casted manually.
+     *
+     * @param DateTimeInterface|null $value
+     * @return DateTimeImmutable|null
+     */
+    private function toDateTimeImmutable(?DateTimeInterface $value): ?DateTimeImmutable
+    {
+        if (true === $value instanceof DateTime) {
+            $value = DateTimeImmutable::createFromMutable($value);
+        }
+
+        return $value;
+    }
 }
diff --git a/Model/ResourceValue.php b/Model/ResourceValue.php
index 72dc831..8eaa7c5 100644
--- a/Model/ResourceValue.php
+++ b/Model/ResourceValue.php
@@ -11,7 +11,7 @@
 
 namespace FSi\Bundle\ResourceRepositoryBundle\Model;
 
-Interface ResourceValue
+interface ResourceValue
 {
     /**
      * @param string $key
diff --git a/Repository/MapBuilder.php b/Repository/MapBuilder.php
index 61e7fa8..8d92218 100644
--- a/Repository/MapBuilder.php
+++ b/Repository/MapBuilder.php
@@ -95,7 +95,7 @@ protected function recursiveParseRawMap(?array $rawMap = [], ?string $parentPath
     {
         $map = [];
 
-        if (!is_array($rawMap)) {
+        if (false === is_array($rawMap)) {
             return $map;
         }
 
@@ -133,7 +133,7 @@ protected function createResource(array $configuration, string $path): ResourceI
     {
         $type = $configuration['type'];
 
-        if (!array_key_exists($type, $this->resourceTypes)) {
+        if (false === array_key_exists($type, $this->resourceTypes)) {
             throw new ConfigurationException(sprintf(
                 '"%s" is not a valid resource type. Try one from: %s',
                 $type,
@@ -183,7 +183,7 @@ protected function validateConfiguration(array $configuration, string $path): vo
             ));
         }
 
-        if (!array_key_exists('type', $configuration)) {
+        if (false === array_key_exists('type', $configuration)) {
             throw new ConfigurationException(sprintf(
                 'Missing "type" declaration in "%s" element configuration',
                 $path
diff --git a/Twig/ResourceRepositoryExtension.php b/Twig/ResourceRepositoryExtension.php
index 30c1fdc..54ff9ea 100644
--- a/Twig/ResourceRepositoryExtension.php
+++ b/Twig/ResourceRepositoryExtension.php
@@ -15,23 +15,18 @@
 use Twig\Extension\AbstractExtension;
 use Twig\TwigFunction;
 
-class ResourceRepositoryExtension extends AbstractExtension
+final class ResourceRepositoryExtension extends AbstractExtension
 {
     /**
      * @var Repository
      */
-    protected $repository;
+    private $repository;
 
     public function __construct(Repository $repository)
     {
         $this->repository = $repository;
     }
 
-    public function getName()
-    {
-        return 'fsi_resource_repository';
-    }
-
     public function getFunctions()
     {
         return [
@@ -40,7 +35,7 @@ public function getFunctions()
             }),
             new TwigFunction('get_resource', function(string $key, $default = null) {
                 $value = $this->repository->get($key);
-                return is_null($value) ? $default : $value;
+                return null !== $value ? $value : $default;
             })
         ];
     }
diff --git a/composer.json b/composer.json
index 81c5191..04afcd7 100644
--- a/composer.json
+++ b/composer.json
@@ -29,7 +29,7 @@
         "fsi/doctrine-extensions-bundle": "^1.0|^2.0",
         "fsi/form-extensions-bundle": "^2.0",
         "phpunit/phpunit": "^7.1",
-        "phpspec/phpspec": "^4.3",
+        "phpspec/phpspec": "^5.0",
         "phpspec/prophecy": "^1.7",
         "sebastian/exporter": "^3.1.0"
     },