diff --git a/chrome/content/zotero/import/mendeley/mendeleyImport.js b/chrome/content/zotero/import/mendeley/mendeleyImport.js
index f2f17b5cf3..9fb4a2447f 100644
--- a/chrome/content/zotero/import/mendeley/mendeleyImport.js
+++ b/chrome/content/zotero/import/mendeley/mendeleyImport.js
@@ -427,6 +427,7 @@ Zotero_Import_Mendeley.prototype._getDocumentCollections = async function (group
 	);
 	for (let row of rows) {
 		let keys = map.get(row.documentId);
+		if (!keys) keys = [];
 		keys.push(folderKeys.get(row.folderId));
 		map.set(row.documentId, keys);
 	}