--- a/toolkit/mozapps/extensions/test/browser/browser_bug567127.js
+++ b/toolkit/mozapps/extensions/test/browser/browser_bug567127.js
@@ -49,16 +49,20 @@ async function checkInstallConfirmation(
 
 add_task(async function test_install_from_file() {
   gManagerWindow = await open_manager("addons://list/extension");
 
   var filePaths = [
                    get_addon_file_url("browser_dragdrop1.xpi"),
                    get_addon_file_url("browser_dragdrop2.xpi"),
                   ];
+  for (let uri of filePaths) {
+    ok(uri.file != null, `Should have file for ${uri.spec}`);
+    ok(uri.file instanceof Ci.nsIFile, `Should have nsIFile for ${uri.spec}`);
+  }
   MockFilePicker.setFiles(filePaths.map(aPath => aPath.file));
 
   // Set handler that executes the core test after the window opens,
   // and resolves the promise when the window closes
   let pInstallURIClosed = checkInstallConfirmation("Drag Drop test 1", "Drag Drop test 2");
 
   gManagerWindow.gViewController.doCommand("cmd_installFromFile");