diff --git a/lib/ManualImageCropEditorWindow.php b/lib/ManualImageCropEditorWindow.php
index 812d8ce..03d572b 100644
--- a/lib/ManualImageCropEditorWindow.php
+++ b/lib/ManualImageCropEditorWindow.php
@@ -36,7 +36,7 @@ public function renderWindow() {
 
 		$imageSizes = get_intermediate_image_sizes();
 
-		$editedSize = in_array($_GET['size'], $imageSizes) ? $_GET['size'] : null;
+		$editedSize = (isset($_GET['size']) && in_array($_GET['size'], $imageSizes)) ? $_GET['size'] : null;
 			
 		$postId = filter_var($_GET['postId'], FILTER_SANITIZE_NUMBER_INT);