diff --git a/app/main.py b/app/main.py
index 777328a..e857f53 100644
--- a/app/main.py
+++ b/app/main.py
@@ -1679,10 +1679,15 @@ def session_view_page(session_id: str, request: Request, user: User = Depends(re
         if (r.ok) {{
           const data = await r.json();
           if (data.ready) {{
-            const f = document.getElementById('app-frame');
-            f.src = iframeSrc;
-            f.style.display = '';
-            return;
+            try {{
+              const probe = await fetch(iframeSrc, {{method:'HEAD', credentials:'include'}});
+              if (probe.status < 500) {{
+                const f = document.getElementById('app-frame');
+                f.src = iframeSrc;
+                f.style.display = '';
+                return;
+              }}
+            }} catch(e) {{}}
           }}
         }}
       }} catch(e) {{}}