diff --git a/chrome/content/zotero/xpcom/storage/webdav.js b/chrome/content/zotero/xpcom/storage/webdav.js
index 31fe47eed..13f64bc86 100644
--- a/chrome/content/zotero/xpcom/storage/webdav.js
+++ b/chrome/content/zotero/xpcom/storage/webdav.js
@@ -526,7 +526,7 @@ Zotero.Sync.Storage.Mode.WebDAV.prototype = {
 		var file = Zotero.getTempDirectory();
 		file.append(item.key + '.zip');
 		Components.utils.importGlobalProperties(["File"]);
-		file = File(file);
+		file = new File(file);
 		
 		var uri = this._getItemURI(item);
 		
diff --git a/chrome/content/zotero/xpcom/storage/zfs.js b/chrome/content/zotero/xpcom/storage/zfs.js
index 4483ecec0..43ee3744b 100644
--- a/chrome/content/zotero/xpcom/storage/zfs.js
+++ b/chrome/content/zotero/xpcom/storage/zfs.js
@@ -618,7 +618,7 @@ Zotero.Sync.Storage.Mode.ZFS.prototype = {
 		var file = yield this._getUploadFile(item);
 		
 		Components.utils.importGlobalProperties(["File"]);
-		file = File(file);
+		file = new File(file);
 		
 		var blob = new Blob([params.prefix, file, params.suffix]);