diff --git a/webapp/src/Command/CheckDatabaseConfigurationDefaultValuesCommand.php b/webapp/src/Command/CheckDatabaseConfigurationDefaultValuesCommand.php
index 5fe5e243bc..f213fdaede 100644
--- a/webapp/src/Command/CheckDatabaseConfigurationDefaultValuesCommand.php
+++ b/webapp/src/Command/CheckDatabaseConfigurationDefaultValuesCommand.php
@@ -15,7 +15,7 @@
 )]
 class CheckDatabaseConfigurationDefaultValuesCommand extends Command
 {
-    public function __construct(protected readonly ConfigurationService $config, string $name = null)
+    public function __construct(protected readonly ConfigurationService $config, ?string $name = null)
     {
         parent::__construct($name);
     }
diff --git a/webapp/src/Command/ImportEventFeedCommand.php b/webapp/src/Command/ImportEventFeedCommand.php
index f8060805c4..eee70cc16e 100644
--- a/webapp/src/Command/ImportEventFeedCommand.php
+++ b/webapp/src/Command/ImportEventFeedCommand.php
@@ -43,7 +43,7 @@ public function __construct(
         protected readonly TokenStorageInterface $tokenStorage,
         protected readonly ?Profiler $profiler,
         protected readonly ExternalContestSourceService $sourceService,
-        string $name = null
+        ?string $name = null
     ) {
         parent::__construct($name);
     }
diff --git a/webapp/src/Command/ScoreboardMergeCommand.php b/webapp/src/Command/ScoreboardMergeCommand.php
index 0533212c04..c75cd5f8bb 100644
--- a/webapp/src/Command/ScoreboardMergeCommand.php
+++ b/webapp/src/Command/ScoreboardMergeCommand.php
@@ -52,7 +52,7 @@ public function __construct(
         protected readonly RouterInterface $router,
         #[Autowire('%kernel.project_dir%')]
         protected readonly string $projectDir,
-        string $name = null
+        ?string $name = null
     ) {
         parent::__construct($name);
     }
diff --git a/webapp/src/Config/Loader/YamlConfigLoader.php b/webapp/src/Config/Loader/YamlConfigLoader.php
index af65092ab0..e7bd28c6ce 100644
--- a/webapp/src/Config/Loader/YamlConfigLoader.php
+++ b/webapp/src/Config/Loader/YamlConfigLoader.php
@@ -15,12 +15,12 @@ class YamlConfigLoader extends FileLoader
     /**
      * @return mixed
      */
-    public function load(mixed $resource, string $type = null)
+    public function load(mixed $resource, ?string $type = null)
     {
         return Yaml::parse(file_get_contents($resource));
     }
 
-    public function supports($resource, string $type = null): bool
+    public function supports($resource, ?string $type = null): bool
     {
         return is_string($resource) &&
             pathinfo($resource, PATHINFO_EXTENSION) === 'yaml';
diff --git a/webapp/src/Controller/API/JudgementTypeController.php b/webapp/src/Controller/API/JudgementTypeController.php
index a20a3fd350..fe4bb3ae81 100644
--- a/webapp/src/Controller/API/JudgementTypeController.php
+++ b/webapp/src/Controller/API/JudgementTypeController.php
@@ -83,7 +83,7 @@ public function singleAction(Request $request, string $id): JudgementType
      *
      * @return JudgementType[]
      */
-    protected function getJudgementTypes(array $filteredOn = null): array
+    protected function getJudgementTypes(?array $filteredOn = null): array
     {
         $verdicts = $this->dj->getVerdicts(mergeExternal: true);
 
diff --git a/webapp/src/Controller/API/ProblemController.php b/webapp/src/Controller/API/ProblemController.php
index ecfab13749..a281237137 100644
--- a/webapp/src/Controller/API/ProblemController.php
+++ b/webapp/src/Controller/API/ProblemController.php
@@ -102,6 +102,7 @@ public function addProblemsAction(Request $request): array
         // Note: we read the JSON as YAML, since any JSON is also YAML and this allows us
         // to import files with YAML inside them that match the JSON format
         $data = Yaml::parseFile($file->getRealPath(), Yaml::PARSE_DATETIME);
+        $messages = [];
         if ($this->importExportService->importProblemsData($contest, $data, $ids, $messages)) {
             return $ids;
         }
diff --git a/webapp/src/Controller/Jury/ImportExportController.php b/webapp/src/Controller/Jury/ImportExportController.php
index 1effc3b10c..c489a9da2a 100644
--- a/webapp/src/Controller/Jury/ImportExportController.php
+++ b/webapp/src/Controller/Jury/ImportExportController.php
@@ -245,6 +245,7 @@ public function indexAction(Request $request): Response
                 $this->addFlash('danger', "Parse error in YAML/JSON file (" . $file->getClientOriginalName() . "): " . $e->getMessage());
                 return $this->redirectToRoute('jury_import_export');
             }
+            $messages = [];
             if ($this->importExportService->importProblemsData($problemsImportForm->get('contest')->getData(), $data, $ids, $messages)) {
                 $this->addFlash('success',
                     sprintf('The file %s is successfully imported.', $file->getClientOriginalName()));
diff --git a/webapp/src/Entity/Balloon.php b/webapp/src/Entity/Balloon.php
index bb4960f83f..70a8bef73e 100644
--- a/webapp/src/Entity/Balloon.php
+++ b/webapp/src/Entity/Balloon.php
@@ -60,7 +60,7 @@ public function getDone(): bool
         return $this->done;
     }
 
-    public function setSubmission(Submission $submission = null): Balloon
+    public function setSubmission(?Submission $submission = null): Balloon
     {
         $this->submission = $submission;
         return $this;
diff --git a/webapp/src/Entity/ExecutableFile.php b/webapp/src/Entity/ExecutableFile.php
index 28202fdf42..0fa7233b6f 100644
--- a/webapp/src/Entity/ExecutableFile.php
+++ b/webapp/src/Entity/ExecutableFile.php
@@ -89,7 +89,7 @@ public function getRank(): int
         return $this->rank;
     }
 
-    public function setImmutableExecutable(ImmutableExecutable $immutableExecutable = null): ExecutableFile
+    public function setImmutableExecutable(?ImmutableExecutable $immutableExecutable = null): ExecutableFile
     {
         $this->immutableExecutable = $immutableExecutable;
         return $this;
diff --git a/webapp/src/Entity/Team.php b/webapp/src/Entity/Team.php
index d485103d6e..b08b485446 100644
--- a/webapp/src/Entity/Team.php
+++ b/webapp/src/Entity/Team.php
@@ -417,7 +417,7 @@ public function setClearPhoto(bool $clearPhoto): Team
         return $this;
     }
 
-    public function setAffiliation(TeamAffiliation $affiliation = null): Team
+    public function setAffiliation(?TeamAffiliation $affiliation = null): Team
     {
         $this->affiliation = $affiliation;
         return $this;
diff --git a/webapp/src/Security/DOMJudgeIPAuthenticator.php b/webapp/src/Security/DOMJudgeIPAuthenticator.php
index 62ebb65e59..507d481142 100644
--- a/webapp/src/Security/DOMJudgeIPAuthenticator.php
+++ b/webapp/src/Security/DOMJudgeIPAuthenticator.php
@@ -139,7 +139,7 @@ public function onAuthenticationFailure(Request $request, AuthenticationExceptio
         return null;
     }
 
-    public function start(Request $request, AuthenticationException $authException = null): Response
+    public function start(Request $request, ?AuthenticationException $authException = null): Response
     {
         // If this is the guard that fails/is configured to allow access as the entry_point
         // send the user a basic auth dialog, as that's probably what they're expecting.
diff --git a/webapp/src/Service/ImportExportService.php b/webapp/src/Service/ImportExportService.php
index be947c60a0..6e175668f3 100644
--- a/webapp/src/Service/ImportExportService.php
+++ b/webapp/src/Service/ImportExportService.php
@@ -151,7 +151,7 @@ protected function convertImportedTime(array $fields, array $data, ?string &$err
         return $time instanceof DateTime ? DateTimeImmutable::createFromMutable($time) : $time;
     }
 
-    public function importContestData(mixed $data, ?string &$errorMessage = null, string &$cid = null): bool
+    public function importContestData(mixed $data, ?string &$errorMessage = null, ?string &$cid = null): bool
     {
         if (empty($data) || !is_array($data)) {
             $errorMessage = 'Error parsing YAML file.';
@@ -327,7 +327,7 @@ public function importContestData(mixed $data, ?string &$errorMessage = null, st
      * @param string[]|null $ids
      * @param array<string, string[]> $messages
      */
-    public function importProblemsData(Contest $contest, array $problems, array &$ids = null, ?array &$messages = []): bool
+    public function importProblemsData(Contest $contest, array $problems, ?array &$ids = null, array &$messages = []): bool
     {
         // For problemset.yaml the root key is called `problems`, so handle that case
         // TODO: Move this check away to make the $problems array shape easier
diff --git a/webapp/src/Service/ScoreboardService.php b/webapp/src/Service/ScoreboardService.php
index e6f2ea208b..35ab9f15cc 100644
--- a/webapp/src/Service/ScoreboardService.php
+++ b/webapp/src/Service/ScoreboardService.php
@@ -950,7 +950,7 @@ public function getScoreboardTwigData(
      * Get the teams to display on the scoreboard.
      * @return Team[]
      */
-    protected function getTeams(Contest $contest, bool $jury = false, Filter $filter = null): array
+    protected function getTeams(Contest $contest, bool $jury = false, ?Filter $filter = null): array
     {
         $queryBuilder = $this->em->createQueryBuilder()
             ->from(Team::class, 't', 't.teamid')