diff --git a/src/UploadBehavior.php b/src/UploadBehavior.php
index 903c326..eedf18d 100644
--- a/src/UploadBehavior.php
+++ b/src/UploadBehavior.php
@@ -546,9 +546,10 @@ protected function getUploadInstance($attribute)
                 }
             }
         }
-        if (is_object($file)) {
+        $files = null;
+        if (isset($file) && is_object($file)) {
             $files = [$file];
-        } else if (is_array($file)) {
+        } else if (isset($file) && is_array($file)) {
             $files = $file;
         }
         if (isset($files) && is_array($files)) {