diff --git a/src/pocketmine/Player.php b/src/pocketmine/Player.php
index a86a11b3d..23c031433 100644
--- a/src/pocketmine/Player.php
+++ b/src/pocketmine/Player.php
@@ -2780,6 +2780,9 @@ class Player extends Human implements CommandSender, ChunkLoader, IPlayer{
 		$pk = new ModalFormRequestPacket();
 		$pk->formId = $id;
 		$pk->formData = json_encode($form);
+		if($pk->formData === false){
+			throw new \InvalidArgumentException("Failed to encode form JSON: " . json_last_error_msg());
+		}
 		if($this->sendDataPacket($pk)){
 			$this->forms[$id] = $form;
 		}