diff --git a/website/src/actions/timetables.test.ts b/website/src/actions/timetables.test.ts
index 8d253ab282..87f78daccd 100644
--- a/website/src/actions/timetables.test.ts
+++ b/website/src/actions/timetables.test.ts
@@ -53,12 +53,13 @@ describe('fillTimetableBlanks', () => {
   const moduleBank = { modules: { CS1010S, CS3216 } };
   const timetablesState = (semester: Semester, timetable: SemTimetableConfig) => ({
     lessons: { [semester]: timetable },
+    customisedModules: { [semester]: [] },
   });
   const semester = 1;
   const action = actions.validateTimetable(semester);
 
   test('do nothing if timetable is already full', () => {
-    const timetable = {
+    const timetable: SemTimetableConfig = {
       CS1010S: {
         Lecture: ['1'],
         Tutorial: ['1'],
@@ -66,6 +67,7 @@ describe('fillTimetableBlanks', () => {
       },
     };
 
+    // TODO(zwliew): Correctly type all the `state: any` declarations in this function and the rest of the codebase.
     const state: any = { timetables: timetablesState(semester, timetable), moduleBank };
     const dispatch = jest.fn();
     action(dispatch, () => state);
@@ -74,7 +76,7 @@ describe('fillTimetableBlanks', () => {
   });
 
   test('fill missing lessons with randomly generated modules', () => {
-    const timetable = {
+    const timetable: SemTimetableConfig = {
       CS1010S: {
         Lecture: ['1'],
         Tutorial: ['1'],
diff --git a/website/src/actions/timetables.ts b/website/src/actions/timetables.ts
index c1a73c833c..ce7763badc 100644
--- a/website/src/actions/timetables.ts
+++ b/website/src/actions/timetables.ts
@@ -236,14 +236,9 @@ export function validateTimetable(semester: Semester) {
       const module = moduleBank.modules[moduleCode];
       if (!module) return;
 
-      // If the module is customised, we do not validate it
-      if (
-        timetables.customisedModules &&
-        timetables.customisedModules[semester] &&
-        timetables.customisedModules[semester].includes(moduleCode)
-      ) {
-        return;
-      }
+      // Do not validate customised modules.
+      if (timetables.customisedModules[semester]?.includes(moduleCode)) return;
+
       const [validatedLessonConfig, changedLessonTypes] = validateModuleLessons(
         semester,
         lessonConfig,
diff --git a/website/src/types/reducers.ts b/website/src/types/reducers.ts
index 406c7bc658..24bb6b7389 100644
--- a/website/src/types/reducers.ts
+++ b/website/src/types/reducers.ts
@@ -113,7 +113,7 @@ export type SettingsState = {
 export type ColorMapping = { [moduleCode: string]: ColorIndex };
 export type SemesterColorMap = { [semester: string]: ColorMapping };
 export type HiddenModulesMap = { [semester: string]: ModuleCode[] };
-export type CustomisedModulesMap = { [semester: string]: ModuleCode[] };
+export type CustomisedModulesMap = { [semester: string]: ModuleCode[] | undefined };
 
 export type TimetablesState = {
   readonly lessons: TimetableConfig;
diff --git a/website/src/views/timetable/TimetableContent.tsx b/website/src/views/timetable/TimetableContent.tsx
index a11d98b873..463f506022 100644
--- a/website/src/views/timetable/TimetableContent.tsx
+++ b/website/src/views/timetable/TimetableContent.tsx
@@ -509,6 +509,7 @@ function mapStateToProps(state: StoreState, ownProps: OwnProps) {
   const { semester, timetable } = ownProps;
   const { modules } = state.moduleBank;
   const timetableWithLessons = hydrateSemTimetableWithLessons(timetable, modules, semester);
+  // TODO(zwliew): fix the type signature of state.timetables.hidden[semester]
   const hiddenInTimetable = state.timetables.hidden[semester] || [];
 
   return {
@@ -518,7 +519,7 @@ function mapStateToProps(state: StoreState, ownProps: OwnProps) {
     modules,
     activeLesson: state.app.activeLesson,
     customiseModule: state.app.customiseModule,
-    customisedModules: state.timetables.customisedModules[semester],
+    customisedModules: state.timetables.customisedModules[semester] ?? [],
     timetableOrientation: state.theme.timetableOrientation,
     showTitle: state.theme.showTitle,
     hiddenInTimetable,