diff --git a/src/editor/model.js b/src/editor/model.js
index 759e13aabb..580085975f 100644
--- a/src/editor/model.js
+++ b/src/editor/model.js
@@ -18,7 +18,7 @@ limitations under the License.
 import {diffAtCaret, diffDeletion} from "./diff";
 
 export default class EditorModel {
-    constructor(parts, partCreator, updateCallback) {
+    constructor(parts, partCreator, updateCallback = null) {
         this._parts = parts;
         this._partCreator = partCreator;
         this._activePartIdx = null;
diff --git a/src/editor/parts.js b/src/editor/parts.js
index 2a6ad81b9b..d3569a7347 100644
--- a/src/editor/parts.js
+++ b/src/editor/parts.js
@@ -363,7 +363,7 @@ export function autoCompleteCreator(getAutocompleterComponent, updateQuery) {
 }
 
 export class PartCreator {
-    constructor(room, client, autoCompleteCreator) {
+    constructor(room, client, autoCompleteCreator = null) {
         this._room = room;
         this._client = client;
         this._autoCompleteCreator = {create: autoCompleteCreator && autoCompleteCreator(this)};