diff --git a/packages/moleculer-db-adapter-mongo/src/index.js b/packages/moleculer-db-adapter-mongo/src/index.js
index 962dfe70..98f88159 100644
--- a/packages/moleculer-db-adapter-mongo/src/index.js
+++ b/packages/moleculer-db-adapter-mongo/src/index.js
@@ -25,7 +25,7 @@ class MongoDbAdapter {
 	 */
 	constructor(uri, opts, dbName) {
 		this.uri = uri,
-		this.opts = opts;
+		this.opts = opts || {};
 		this.dbName = dbName;
 	}
 
@@ -257,7 +257,7 @@ class MongoDbAdapter {
 	entityToObject(entity) {
 		let json = Object.assign({}, entity);
 		if (entity._id)
-			json._id = this.objectIDToString(entity._id);
+			json[this.opts.jsonId ? this.opts.jsonId : "_id"] = this.objectIDToString(entity._id);
 		return json;
 	}