diff --git a/frontend/src/components/SettingsForm/index.tsx b/frontend/src/components/SettingsForm/index.tsx
index 9f211e24..4417529e 100644
--- a/frontend/src/components/SettingsForm/index.tsx
+++ b/frontend/src/components/SettingsForm/index.tsx
@@ -129,7 +129,10 @@ const SettingsForm = ({
               onChange={(e) => {
                 const fontSize = e.target.value;
                 setSettings({ ...settings, fontSize });
-                systemClient.setCookie(`settings_fontsize_${settings.frontend}`, fontSize);
+                systemClient.setCookie(
+                  `settings_fontsize_${settings.frontend}`,
+                  fontSize.toString(),
+                );
               }}
               valueLabelDisplay='off'
               marks={fontSizes.map(({ label, value }) => ({
diff --git a/frontend/src/services/System/SystemNativeClient/index.ts b/frontend/src/services/System/SystemNativeClient/index.ts
index 15a149a7..c27770dc 100644
--- a/frontend/src/services/System/SystemNativeClient/index.ts
+++ b/frontend/src/services/System/SystemNativeClient/index.ts
@@ -22,8 +22,9 @@ class SystemNativeClient implements SystemClient {
     });
   };
 
-  public getCookie: (key: string) => string | undefined = (key) => {
-    return window.NativeRobosats?.cookies[key];
+  public getCookie: (key: string) => string = (key) => {
+    const cookie = window.NativeRobosats?.cookies[key];
+    return cookie === null || cookie === undefined ? '' : cookie;
   };
 
   public setCookie: (key: string, value: string) => void = (key, value) => {
diff --git a/frontend/src/services/System/SystemWebClient/index.ts b/frontend/src/services/System/SystemWebClient/index.ts
index 2720f724..3aab1a3c 100644
--- a/frontend/src/services/System/SystemWebClient/index.ts
+++ b/frontend/src/services/System/SystemWebClient/index.ts
@@ -27,7 +27,7 @@ class SystemWebClient implements SystemClient {
     }
   };
 
-  public getCookie: (key: string) => string | undefined = (key) => {
+  public getCookie: (key: string) => string = (key) => {
     let cookieValue = null;
     if (document.cookie && document.cookie !== '') {
       const cookies = document.cookie.split(';');