diff --git a/chrome/content/zotero/xpcom/data/collections.js b/chrome/content/zotero/xpcom/data/collections.js
index 3fa2920b6b..857161b7d4 100644
--- a/chrome/content/zotero/xpcom/data/collections.js
+++ b/chrome/content/zotero/xpcom/data/collections.js
@@ -179,12 +179,24 @@ Zotero.Collections = function() {
 			let tree = [ids[0]];
 			let keep = [ids[0]];
 			let id = ids.shift();
+			let seen = new Set([id]);
 			while (true) {
 				let c = Zotero.Collections.get(id);
 				let parentID = c.parentID;
 				if (!parentID) {
 					break;
 				}
+				// Avoid an infinite loop if collections are incorrectly nested within each other
+				if (seen.has(parentID)) {
+					throw new Zotero.Error(
+						"Incorrectly nested collections",
+						Zotero.Error.ERROR_INVALID_COLLECTION_NESTING,
+						{
+							collectionID: id
+						}
+					);
+				}
+				seen.add(parentID);
 				tree.push(parentID);
 				// If parent is in list, remove it
 				let pos = ids.indexOf(parentID);
diff --git a/chrome/content/zotero/xpcom/error.js b/chrome/content/zotero/xpcom/error.js
index d1305676d2..3c6471eef9 100644
--- a/chrome/content/zotero/xpcom/error.js
+++ b/chrome/content/zotero/xpcom/error.js
@@ -51,6 +51,7 @@ Zotero.Error.ERROR_ZFS_UPLOAD_QUEUE_LIMIT = 6;
 Zotero.Error.ERROR_ZFS_FILE_EDITING_DENIED = 7;
 Zotero.Error.ERROR_INVALID_ITEM_TYPE = 8;
 Zotero.Error.ERROR_USER_NOT_AVAILABLE = 9;
+Zotero.Error.ERROR_INVALID_COLLECTION_NESTING = 10;
 //Zotero.Error.ERROR_SYNC_EMPTY_RESPONSE_FROM_SERVER = 6;
 //Zotero.Error.ERROR_SYNC_INVALID_RESPONSE_FROM_SERVER = 7;
 
diff --git a/chrome/content/zotero/xpcom/sync/syncLocal.js b/chrome/content/zotero/xpcom/sync/syncLocal.js
index f0b212862b..decffc7e6d 100644
--- a/chrome/content/zotero/xpcom/sync/syncLocal.js
+++ b/chrome/content/zotero/xpcom/sync/syncLocal.js
@@ -542,7 +542,23 @@ Zotero.Sync.Data.Local = {
 		
 		// Sort descendent collections last
 		if (objectType == 'collection') {
-			ids = Zotero.Collections.sortByLevel(ids);
+			try {
+				ids = Zotero.Collections.sortByLevel(ids);
+			}
+			catch (e) {
+				Zotero.logError(e);
+				// If collections were incorrectly nested, fix and try again
+				if (e instanceof Zotero.Error && e.error == Zotero.Error.ERROR_INVALID_COLLECTION_NESTING) {
+					let c = Zotero.Collections.get(e.collectionID);
+					Zotero.debug(`Removing parent collection ${c.parentKey} from collection ${c.key}`);
+					c.parentID = null;
+					yield c.saveTx();
+					return this.getUnsynced(...arguments);
+				}
+				else {
+					throw e;
+				}
+			}
 		}
 		
 		return ids;
diff --git a/test/tests/syncLocalTest.js b/test/tests/syncLocalTest.js
index 60e42fa478..3e95250009 100644
--- a/test/tests/syncLocalTest.js
+++ b/test/tests/syncLocalTest.js
@@ -466,6 +466,33 @@ describe("Zotero.Sync.Data.Local", function() {
 	})
 	
 	
+	describe("#getUnsynced()", function () {
+		it("should correct incorrectly nested collections", async function () {
+			var c1 = await createDataObject('collection');
+			var c2 = await createDataObject('collection');
+			
+			c1.parentID = c2.id;
+			await c1.saveTx();
+			
+			await Zotero.DB.queryAsync(
+				"UPDATE collections SET parentCollectionID=? WHERE collectionID=?",
+				[
+					c1.id,
+					c2.id
+				]
+			);
+			await c2.reload(['primaryData'], true);
+			
+			var ids = await Zotero.Sync.Data.Local.getUnsynced('collection', Zotero.Libraries.userLibraryID);
+			
+			// One of the items should still be the parent of the other, though which one is undefined
+			assert.isTrue(
+				(c1.parentID == c2.id && !c2.parentID) || (c2.parentID == c1.id && !c1.parentID)
+			);
+		});
+	});
+	
+	
 	describe("#processObjectsFromJSON()", function () {
 		var types = Zotero.DataObjectUtilities.getTypes();