diff --git a/src/bootstrap.js b/src/bootstrap.js
index f8cc287..b376ebf 100644
--- a/src/bootstrap.js
+++ b/src/bootstrap.js
@@ -43,7 +43,7 @@ const startCore = () => {
       bw.webContents.executeJavaScript(readFileSync(join(__dirname, 'mainWindow.js'), 'utf8')
         .replaceAll('<hash>', hash || 'custom')
         .replaceAll('<notrack>', oaConfig.noTrack)
-        .replace('<css>', oaConfig.css.replaceAll('`', '\\`').replaceAll('\\', '\\\\')));
+        .replace('<css>', (oaConfig.css ?? '').replaceAll('`', '\\`').replaceAll('\\', '\\\\')));
 
       if (oaConfig.js) bw.webContents.executeJavaScript(oaConfig.js);
     });