diff --git a/lib/Command/ETL.php b/lib/Command/ETL.php
index bccd4042..05806e80 100644
--- a/lib/Command/ETL.php
+++ b/lib/Command/ETL.php
@@ -49,11 +49,12 @@ public function __construct(ETLService $etlService) {
 		);
 	}
 
-	protected function execute(InputInterface $input, OutputInterface $output) {
+	protected function execute(InputInterface $input, OutputInterface $output): int {
 		$max = $input->getOption('max');
 		$this->etlService->extractAndTransform(
 			(int)$max,
 			$output
 		);
+		return 0;
 	}
 }
diff --git a/lib/Command/Optimize.php b/lib/Command/Optimize.php
index 07e8e35e..0884d741 100644
--- a/lib/Command/Optimize.php
+++ b/lib/Command/Optimize.php
@@ -67,7 +67,7 @@ public function __construct(OptimizerService $optimizer) {
 		$this->registerStatsOption();
 	}
 
-	protected function execute(InputInterface $input, OutputInterface $output) {
+	protected function execute(InputInterface $input, OutputInterface $output): int {
 		if (extension_loaded('xdebug')) {
 			$output->writeln('<comment>XDebug is active. This will slow down the training processes.</comment>');
 		}
diff --git a/lib/Command/Predict.php b/lib/Command/Predict.php
index 46393f9c..8bfb84c7 100644
--- a/lib/Command/Predict.php
+++ b/lib/Command/Predict.php
@@ -66,7 +66,7 @@ public function __construct(EstimatorService $estimatorService) {
 		);
 	}
 
-	protected function execute(InputInterface $input, OutputInterface $output) {
+	protected function execute(InputInterface $input, OutputInterface $output): int {
 		$uid = $input->getArgument('uid');
 		$ip = $input->getArgument('ip');
 		$modelId = $input->getArgument('model');
diff --git a/lib/Command/Seed.php b/lib/Command/Seed.php
index cb5a5d61..d7d568e0 100644
--- a/lib/Command/Seed.php
+++ b/lib/Command/Seed.php
@@ -57,7 +57,7 @@ public function __construct(IConfig $config,
 		$this->config = $config;
 	}
 
-	protected function execute(InputInterface $input, OutputInterface $output) {
+	protected function execute(InputInterface $input, OutputInterface $output): int {
 		if ($this->config->getSystemValueBool('debug', false) === false) {
 			$output->writeln("<error>This command is meant for development purposes.</error> Enable debug mode and try again if you know what you are doing.");
 			return 1;
diff --git a/lib/Command/Train.php b/lib/Command/Train.php
index 13997d61..8a4882b3 100644
--- a/lib/Command/Train.php
+++ b/lib/Command/Train.php
@@ -130,7 +130,7 @@ public function __construct(DataLoader $loader,
 		$this->registerStatsOption();
 	}
 
-	protected function execute(InputInterface $input, OutputInterface $output) {
+	protected function execute(InputInterface $input, OutputInterface $output): int {
 		$strategy = $input->getOption('v6') ? new IpV6Strategy() : new Ipv4Strategy();
 		$config = $strategy->getDefaultMlpConfig();
 		if ($input->getOption('epochs') !== null) {