diff --git a/lib/Segment.php b/lib/Segment.php
index 2e1aee0..875f389 100644
--- a/lib/Segment.php
+++ b/lib/Segment.php
@@ -53,7 +53,9 @@ public static function identify(array $message) {
   public static function group(array $message) {
     self::checkClient();
     $groupId = !empty($message["groupId"]);
-    self::assert($groupId, "Segment::group() expects groupId");
+    $userId = !(array_key_exists('userID', $message) && strlen((string) $message['userID']) > 0);
+    
+    self::assert($groupId && $userId, "Segment::group() expects userId and groupId");
     self::validate($message, "group");
 
     return self::$client->group($message);
@@ -93,8 +95,8 @@ public static function screen(array $message) {
    */
   public static function alias(array $message) {
     self::checkClient();
-    $userId = !empty($message["userId"]);
-    $previousId = !empty($message["previousId"]);
+    $userId = !(array_key_exists('userID', $message) && strlen((string) $message['userID']) > 0);
+    $previousId = !(array_key_exists('previousId', $message) && strlen((string) $message['previousId']) > 0);
     self::assert($userId && $previousId, "Segment::alias() requires both userId and previousId");
 
     return self::$client->alias($message);
@@ -107,7 +109,7 @@ public static function alias(array $message) {
    * @param string $type
    */
   public static function validate($msg, $type){
-    $userId = !empty($msg["userId"]);
+    $userId = !(array_key_exists('userID', $msg) && strlen((string) $msg['userID']) > 0);
     $anonId = !empty($msg["anonymousId"]);
     self::assert($userId || $anonId, "Segment::${type}() requires userId or anonymousId");
   }