diff --git a/composer.json b/composer.json
index f4aa2af78..86bf002ec 100644
--- a/composer.json
+++ b/composer.json
@@ -30,7 +30,7 @@
       "pocketmine/raklib": "^0.12.0",
       "pocketmine/spl": "^0.3.0",
       "pocketmine/binaryutils": "^0.1.0",
-      "pocketmine/nbt": "^0.2.0",
+      "pocketmine/nbt": "^0.2.1",
       "pocketmine/math": "^0.2.0",
       "pocketmine/snooze": "^0.1.0"
    },
diff --git a/composer.lock b/composer.lock
index db547f14e..a35c19934 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "85fe0f9aca77848b0f753dbc28815c79",
+    "content-hash": "9ff16274822f181c2ed533797d2e22f7",
     "packages": [
         {
             "name": "fgrosse/phpasn1",
@@ -217,16 +217,16 @@
         },
         {
             "name": "pocketmine/nbt",
-            "version": "0.2.0",
+            "version": "0.2.1",
             "source": {
                 "type": "git",
                 "url": "https://github.com/pmmp/NBT.git",
-                "reference": "da19487ff92f6f7a16b5ce8894132bb1d1e9ea0c"
+                "reference": "a4ee39f313c6870153fb7c7e513b211217f7daf8"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/pmmp/NBT/zipball/da19487ff92f6f7a16b5ce8894132bb1d1e9ea0c",
-                "reference": "da19487ff92f6f7a16b5ce8894132bb1d1e9ea0c",
+                "url": "https://api.github.com/repos/pmmp/NBT/zipball/a4ee39f313c6870153fb7c7e513b211217f7daf8",
+                "reference": "a4ee39f313c6870153fb7c7e513b211217f7daf8",
                 "shasum": ""
             },
             "require": {
@@ -250,10 +250,10 @@
             ],
             "description": "PHP library for working with Named Binary Tags",
             "support": {
-                "source": "https://github.com/pmmp/NBT/tree/0.2.0",
+                "source": "https://github.com/pmmp/NBT/tree/0.2.1",
                 "issues": "https://github.com/pmmp/NBT/issues"
             },
-            "time": "2018-06-13T09:56:00+00:00"
+            "time": "2018-09-04T10:36:02+00:00"
         },
         {
             "name": "pocketmine/raklib",
diff --git a/src/pocketmine/Player.php b/src/pocketmine/Player.php
index c5e46ff37..4f7df34d5 100644
--- a/src/pocketmine/Player.php
+++ b/src/pocketmine/Player.php
@@ -3281,6 +3281,9 @@ class Player extends Human implements CommandSender, ChunkLoader, IPlayer{
 	 * @param bool      $isPermanent Prevents the window being removed if true.
 	 *
 	 * @return int
+	 *
+	 * @throws \InvalidArgumentException if a forceID which is already in use is specified
+	 * @throws \InvalidStateException if trying to add a window without forceID when no slots are free
 	 */
 	public function addWindow(Inventory $inventory, int $forceId = null, bool $isPermanent = false) : int{
 		if(($id = $this->getWindowId($inventory)) !== ContainerIds::NONE){
@@ -3288,10 +3291,21 @@ class Player extends Human implements CommandSender, ChunkLoader, IPlayer{
 		}
 
 		if($forceId === null){
-			$this->windowCnt = $cnt = max(ContainerIds::FIRST, ++$this->windowCnt % ContainerIds::LAST);
+			$cnt = $this->windowCnt;
+			do{
+				$cnt = max(ContainerIds::FIRST, ($cnt + 1) % ContainerIds::LAST);
+				if($cnt === $this->windowCnt){ //wraparound, no free slots
+					throw new \InvalidStateException("No free window IDs found");
+				}
+			}while(isset($this->windowIndex[$cnt]));
+			$this->windowCnt = $cnt;
 		}else{
 			$cnt = $forceId;
+			if(isset($this->windowIndex[$cnt])){
+				throw new \InvalidArgumentException("Requested force ID $forceId already in use");
+			}
 		}
+
 		$this->windowIndex[$cnt] = $inventory;
 		$this->windows[spl_object_hash($inventory)] = $cnt;
 		if($inventory->open($this)){