diff --git a/chrome/content/zotero/xpcom/storage/storageEngine.js b/chrome/content/zotero/xpcom/storage/storageEngine.js
index 4804abc92d..224998ced8 100644
--- a/chrome/content/zotero/xpcom/storage/storageEngine.js
+++ b/chrome/content/zotero/xpcom/storage/storageEngine.js
@@ -227,11 +227,11 @@ Zotero.Sync.Storage.Engine.prototype.start = async function () {
 		let failed = 0;
 		
 		for (let p of results) {
-			if (p.isFulfilled()) {
+			try {
+				await p;
 				succeeded++;
 			}
-			else if (!p.isPending()) {
-				let e = p.reason();
+			catch (e) {
 				if (e instanceof Zotero.HTTP.CancelledException) {
 					Zotero.debug(`File ${type} sync cancelled for ${this.library.name} `
 						+ `(${succeeded} succeeded, ${failed} failed)`);
diff --git a/resource/concurrentCaller.mjs b/resource/concurrentCaller.mjs
index 3d4a11cb70..d060a01f70 100644
--- a/resource/concurrentCaller.mjs
+++ b/resource/concurrentCaller.mjs
@@ -115,19 +115,21 @@ ConcurrentCaller.prototype.add = function (func) {
 	if (Array.isArray(func)) {
 		let promises = [];
 		for (let i = 0; i < func.length; i++) {
-			promises.push(this.start(func[i]).reflect());
+			promises.push(this.start(func[i]));
 		}
-		return Promise.all(promises);
+		return Promise.allSettled(promises).then(() => promises);
 	}
 	
-	if (!this._deferred || !this._deferred.promise.isPending()) {
+	if (!this._deferred) {
 		this._deferred = Promise.defer();
+		let onResolve = () => this._deferred = null;
+		this._deferred.promise.then(onResolve, onResolve);
 	}
 	
 	var deferred = Promise.defer();
 	this._queue.push({
-		func: Promise.method(func),
-		deferred: deferred
+		func,
+		deferred
 	});
 	return deferred.promise;
 }
@@ -151,7 +153,7 @@ ConcurrentCaller.prototype.start = function (func) {
 /**
  * Start processing if not already running and wait for all tasks to complete
  *
- * @return {Promise[]} - An array of promises for all currently queued tasks
+ * @return {Promise<Promise[]>} - An array of promises for all currently queued tasks
  */
 ConcurrentCaller.prototype.runAll = function () {
 	// If nothing queued, return immediately
@@ -163,7 +165,7 @@ ConcurrentCaller.prototype.runAll = function () {
 		var run = this._processNext();
 	}
 	while (run);
-	return this._deferred.promise.return(promises);
+	return this._deferred.promise.then(() => promises);
 }