diff --git a/rector.php b/rector.php
index a81f8384..836677e8 100644
--- a/rector.php
+++ b/rector.php
@@ -15,6 +15,7 @@
 use Rector\Php74\Rector\Closure\ClosureToArrowFunctionRector;
 use Rector\TypeDeclaration\Rector\ArrowFunction\AddArrowFunctionReturnTypeRector;
 use Rector\TypeDeclaration\Rector\ClassMethod\ReturnNeverTypeRector;
+use Rector\TypeDeclaration\Rector\Closure\AddClosureVoidReturnTypeWhereNoReturnRector;
 
 return RectorConfig::configure()
     ->withPaths(['config', 'resources', 'src'])
@@ -33,4 +34,5 @@
         PostIncDecToPreIncDecRector::class,
         NullableCompareToNullRector::class,
         AddArrowFunctionReturnTypeRector::class,
+        AddClosureVoidReturnTypeWhereNoReturnRector::class,
     ]);
diff --git a/src/BackupDestination/BackupCollection.php b/src/BackupDestination/BackupCollection.php
index dc998fdf..b3564010 100644
--- a/src/BackupDestination/BackupCollection.php
+++ b/src/BackupDestination/BackupCollection.php
@@ -16,7 +16,7 @@ public static function createFromFiles(?FileSystem $disk, array $files): self
     {
         return (new static($files))
             ->filter(fn (string $path) => (new File())->isZipFile($disk, $path))
-            ->map(fn (string $path) => new Backup($disk, $path))
+            ->map(fn (string $path): \Spatie\Backup\BackupDestination\Backup => new Backup($disk, $path))
             ->sortByDesc(fn (Backup $backup) => $backup->date()->timestamp)
             ->values();
     }
diff --git a/src/BackupServiceProvider.php b/src/BackupServiceProvider.php
index bf1559e3..92e10d75 100644
--- a/src/BackupServiceProvider.php
+++ b/src/BackupServiceProvider.php
@@ -52,7 +52,7 @@ public function packageRegistered(): void
 
         $this->registerDiscordChannel();
 
-        $this->app->scoped(Config::class, function () {
+        $this->app->scoped(Config::class, function (): \Spatie\Backup\Config\Config {
             return Config::fromArray(config('backup'));
         });
     }
@@ -60,7 +60,7 @@ public function packageRegistered(): void
     protected function registerDiscordChannel(): void
     {
         Notification::resolved(function (ChannelManager $service) {
-            $service->extend('discord', function ($app) {
+            $service->extend('discord', function ($app): \Spatie\Backup\Notifications\Channels\Discord\DiscordChannel {
                 return new DiscordChannel();
             });
         });
diff --git a/src/Commands/ListCommand.php b/src/Commands/ListCommand.php
index ac024e50..7e226eeb 100644
--- a/src/Commands/ListCommand.php
+++ b/src/Commands/ListCommand.php
@@ -39,7 +39,7 @@ protected function displayOverview(Collection $backupDestinationStatuses): stati
     {
         $headers = ['Name', 'Disk', 'Reachable', 'Healthy', '# of backups', 'Newest backup', 'Used storage'];
 
-        $rows = $backupDestinationStatuses->map(function (BackupDestinationStatus $backupDestinationStatus) {
+        $rows = $backupDestinationStatuses->map(function (BackupDestinationStatus $backupDestinationStatus): array {
             return $this->convertToRow($backupDestinationStatus);
         });
 
@@ -83,10 +83,10 @@ public function convertToRow(BackupDestinationStatus $backupDestinationStatus):
     protected function displayFailures(Collection $backupDestinationStatuses): static
     {
         $failed = $backupDestinationStatuses
-            ->filter(function (BackupDestinationStatus $backupDestinationStatus) {
+            ->filter(function (BackupDestinationStatus $backupDestinationStatus): bool {
                 return $backupDestinationStatus->getHealthCheckFailure() !== null;
             })
-            ->map(function (BackupDestinationStatus $backupDestinationStatus) {
+            ->map(function (BackupDestinationStatus $backupDestinationStatus): array {
                 return [
                     $backupDestinationStatus->backupDestination()->backupName(),
                     $backupDestinationStatus->backupDestination()->diskName(),
diff --git a/src/Config/Config.php b/src/Config/Config.php
index b3667476..e99baa30 100644
--- a/src/Config/Config.php
+++ b/src/Config/Config.php
@@ -17,7 +17,7 @@ protected function __construct(
     /** @internal used for testing */
     public static function rebind(): void
     {
-        app()->scoped(Config::class, function () {
+        app()->scoped(Config::class, function (): \Spatie\Backup\Config\Config {
             return self::fromArray(config('backup'));
         });
     }
diff --git a/src/Config/DestinationConfig.php b/src/Config/DestinationConfig.php
index e983edff..9894f727 100644
--- a/src/Config/DestinationConfig.php
+++ b/src/Config/DestinationConfig.php
@@ -17,7 +17,11 @@ protected function __construct(
         public string $filenamePrefix,
         public array $disks,
     ) {
-        if ($compressionLevel > 9 || $compressionLevel < 0) {
+        if ($compressionLevel > 9) {
+            throw InvalidConfig::integerMustBeBetween('compression_level', 0, 9);
+        }
+
+        if ($compressionLevel < 0) {
             throw InvalidConfig::integerMustBeBetween('compression_level', 0, 9);
         }
     }
diff --git a/src/Tasks/Backup/BackupJob.php b/src/Tasks/Backup/BackupJob.php
index ad702f87..ad3c0453 100644
--- a/src/Tasks/Backup/BackupJob.php
+++ b/src/Tasks/Backup/BackupJob.php
@@ -254,7 +254,7 @@ protected function createZipContainingEveryFileInManifest(Manifest $manifest): s
     protected function dumpDatabases(): array
     {
         return $this->dbDumpers
-            ->map(function (DbDumper $dbDumper, $key) {
+            ->map(function (DbDumper $dbDumper, string $key): string {
                 consoleOutput()->info("Dumping database {$dbDumper->getDbName()}...");
 
                 $dbType = mb_strtolower(basename(str_replace('\\', '/', $dbDumper::class)));
diff --git a/src/Tasks/Backup/BackupJobFactory.php b/src/Tasks/Backup/BackupJobFactory.php
index 67e98355..a047f6e0 100644
--- a/src/Tasks/Backup/BackupJobFactory.php
+++ b/src/Tasks/Backup/BackupJobFactory.php
@@ -33,7 +33,7 @@ protected static function createFileSelection(SourceFilesConfig $sourceFiles): F
     protected static function createDbDumpers(array $dbConnectionNames): Collection
     {
         return collect($dbConnectionNames)->mapWithKeys(
-            fn (string $dbConnectionName) => [$dbConnectionName => DbDumperFactory::createFromConnection($dbConnectionName)]
+            fn (string $dbConnectionName): array => [$dbConnectionName => DbDumperFactory::createFromConnection($dbConnectionName)]
         );
     }
 }
diff --git a/src/Tasks/Backup/Zip.php b/src/Tasks/Backup/Zip.php
index 3fd66167..41181481 100644
--- a/src/Tasks/Backup/Zip.php
+++ b/src/Tasks/Backup/Zip.php
@@ -111,13 +111,11 @@ public function add(string|iterable $files, ?string $nameInZip = null): self
             if (is_file($file)) {
                 $this->zipFile->addFile($file, ltrim((string) $nameInZip, DIRECTORY_SEPARATOR));
 
-                if (is_int($compressionMethod)) {
-                    $this->zipFile->setCompressionName(
-                        ltrim($nameInZip ?: $file, DIRECTORY_SEPARATOR),
-                        $compressionMethod,
-                        $compressionLevel
-                    );
-                }
+                $this->zipFile->setCompressionName(
+                    ltrim($nameInZip ?: $file, DIRECTORY_SEPARATOR),
+                    $compressionMethod,
+                    $compressionLevel
+                );
             }
 
             $this->fileCount++;
diff --git a/src/Tasks/Monitor/BackupDestinationStatusFactory.php b/src/Tasks/Monitor/BackupDestinationStatusFactory.php
index 46e47cc0..42c0c6ec 100644
--- a/src/Tasks/Monitor/BackupDestinationStatusFactory.php
+++ b/src/Tasks/Monitor/BackupDestinationStatusFactory.php
@@ -26,7 +26,7 @@ public static function createForMonitorConfig(MonitoredBackupsConfig $monitorCon
     public static function createForSingleMonitor(array $monitorConfig): Collection
     {
         return collect($monitorConfig['disks'])
-            ->map(function ($diskName) use ($monitorConfig) {
+            ->map(function ($diskName) use ($monitorConfig): \Spatie\Backup\Tasks\Monitor\BackupDestinationStatus {
                 $backupDestination = BackupDestination::create($diskName, $monitorConfig['name']);
 
                 return new BackupDestinationStatus($backupDestination, static::buildHealthChecks($monitorConfig));
@@ -40,7 +40,7 @@ public static function createForSingleMonitor(array $monitorConfig): Collection
     protected static function buildHealthChecks(array $monitorConfig): array
     {
         return collect($monitorConfig['healthChecks'])
-            ->map(function ($options, $class) {
+            ->map(function ($options, $class): \Spatie\Backup\Tasks\Monitor\HealthCheck {
                 if (is_int($class)) {
                     $class = $options;
                     $options = [];