diff --git a/packages/client/src/pages/settings/theme.vue b/packages/client/src/pages/settings/theme.vue
index 1bdad3e75..d330e1ba2 100644
--- a/packages/client/src/pages/settings/theme.vue
+++ b/packages/client/src/pages/settings/theme.vue
@@ -97,7 +97,10 @@ const darkThemeId = computed({
 		return darkTheme.value.id;
 	},
 	set(id) {
-		ColdDeviceStorage.set('darkTheme', themes.value.find(x => x.id === id));
+		const t = themes.value.find(x => x.id === id);
+		if (t) { // テーマエディタでテーマを作成したときなどは、themesに反映されないため undefined になる
+			ColdDeviceStorage.set('darkTheme', t);
+		}
 	},
 });
 const lightTheme = ColdDeviceStorage.ref('lightTheme');
@@ -106,7 +109,10 @@ const lightThemeId = computed({
 		return lightTheme.value.id;
 	},
 	set(id) {
-		ColdDeviceStorage.set('lightTheme', themes.value.find(x => x.id === id));
+		const t = themes.value.find(x => x.id === id);
+		if (t) { // テーマエディタでテーマを作成したときなどは、themesに反映されないため undefined になる
+			ColdDeviceStorage.set('lightTheme', t);
+		}
 	},
 });
 const darkMode = computed(defaultStore.makeGetterSetter('darkMode'));
diff --git a/packages/client/src/pages/theme-editor.vue b/packages/client/src/pages/theme-editor.vue
index 44b5a05f2..548e60614 100644
--- a/packages/client/src/pages/theme-editor.vue
+++ b/packages/client/src/pages/theme-editor.vue
@@ -192,7 +192,7 @@ async function saveAs() {
 	theme.name = name;
 	theme.author = `@${$i.username}@${toUnicode(host)}`;
 	if (description) theme.desc = description;
-	addTheme(theme);
+	await addTheme(theme);
 	applyTheme(theme);
 	if (defaultStore.state.darkMode) {
 		ColdDeviceStorage.set('darkTheme', theme);
diff --git a/packages/client/src/store.ts b/packages/client/src/store.ts
index d87e05a4d..503333331 100644
--- a/packages/client/src/store.ts
+++ b/packages/client/src/store.ts
@@ -304,6 +304,14 @@ export class ColdDeviceStorage {
 	}
 
 	public static set<T extends keyof typeof ColdDeviceStorage.default>(key: T, value: typeof ColdDeviceStorage.default[T]): void {
+		// 呼び出し側のバグ等で undefined が来ることがある
+		// undefined を文字列として localStorage に入れると参照する際の JSON.parse でコケて不具合の元になるため無視
+		// eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
+		if (value === undefined) {
+			console.error(`attempt to store undefined value for key '${key}'`);
+			return;
+		}
+
 		localStorage.setItem(PREFIX + key, JSON.stringify(value));
 
 		for (const watcher of this.watchers) {