diff --git a/src/pocketmine/command/CommandMap.php b/src/pocketmine/command/CommandMap.php
index 6b946a6aa..b48ecb815 100644
--- a/src/pocketmine/command/CommandMap.php
+++ b/src/pocketmine/command/CommandMap.php
@@ -37,9 +37,9 @@ interface CommandMap{
 	 * @param Command     $command
 	 * @param string|null $label
 	 *
-	 * @return
+	 * @return bool
 	 */
-	public function register(string $fallbackPrefix, Command $command, string $label = null);
+	public function register(string $fallbackPrefix, Command $command, string $label = null) : bool;
 
 	/**
 	 * @param CommandSender $sender
diff --git a/src/pocketmine/command/SimpleCommandMap.php b/src/pocketmine/command/SimpleCommandMap.php
index 2fef70b6a..6e908c444 100644
--- a/src/pocketmine/command/SimpleCommandMap.php
+++ b/src/pocketmine/command/SimpleCommandMap.php
@@ -137,7 +137,14 @@ class SimpleCommandMap implements CommandMap{
 		}
 	}
 
-	public function register(string $fallbackPrefix, Command $command, string $label = null){
+	/**
+	 * @param string      $fallbackPrefix
+	 * @param Command     $command
+	 * @param string|null $label
+	 *
+	 * @return bool
+	 */
+	public function register(string $fallbackPrefix, Command $command, string $label = null) : bool{
 		if($label === null){
 			$label = $command->getName();
 		}
@@ -163,7 +170,15 @@ class SimpleCommandMap implements CommandMap{
 		return $registered;
 	}
 
-	private function registerAlias(Command $command, $isAlias, $fallbackPrefix, $label){
+	/**
+	 * @param Command $command
+	 * @param bool $isAlias
+	 * @param string $fallbackPrefix
+	 * @param string $label
+	 *
+	 * @return bool
+	 */
+	private function registerAlias(Command $command, bool $isAlias, string $fallbackPrefix, string $label) : bool{
 		$this->knownCommands[$fallbackPrefix . ":" . $label] = $command;
 		if(($command instanceof VanillaCommand or $isAlias) and isset($this->knownCommands[$label])){
 			return false;