diff --git a/app.py b/app.py
index 8acdd07..df59a52 100644
--- a/app.py
+++ b/app.py
@@ -181,7 +181,7 @@ def api_network():
         try:
             return jsonify(json.loads(raw))
         except Exception:
-            pass
+            logger.error('Failed to parse network_snapshot JSON')
     return jsonify({'hosts': {}, 'unifi': [], 'updated': None})
 
 
@@ -193,7 +193,7 @@ def api_links():
         try:
             return jsonify(json.loads(raw))
         except Exception:
-            pass
+            logger.error('Failed to parse link_stats JSON')
     return jsonify({'hosts': {}, 'updated': None})
 
 
@@ -261,9 +261,12 @@ def api_diagnose_start():
     """Start a link diagnostic job. Returns {job_id}."""
     data = request.get_json(silent=True) or {}
     switch_name = (data.get('switch_name') or '').strip()
-    port_idx    = data.get('port_idx')
+    try:
+        port_idx = int(data.get('port_idx'))
+    except (TypeError, ValueError):
+        return jsonify({'error': 'port_idx must be an integer'}), 400
 
-    if not switch_name or port_idx is None:
+    if not switch_name:
         return jsonify({'error': 'switch_name and port_idx required'}), 400
 
     # Look up switch + port in cached link_stats