diff --git a/chrome/content/zotero/xpcom/http.js b/chrome/content/zotero/xpcom/http.js
index f8de939c1..48147429b 100644
--- a/chrome/content/zotero/xpcom/http.js
+++ b/chrome/content/zotero/xpcom/http.js
@@ -864,7 +864,7 @@ Zotero.HTTP = new function() {
 			}
 			
 			// If processor returns a promise, wait for it
-			if (maybePromise.then) {
+			if (maybePromise && maybePromise.then) {
 				maybePromise.then(() => doLoad())
 				.catch(e => {
 					if (exception) {