diff --git a/src/extension_shield/api/main.py b/src/extension_shield/api/main.py
index 3ebbc4c..50da945 100644
--- a/src/extension_shield/api/main.py
+++ b/src/extension_shield/api/main.py
@@ -4035,8 +4035,13 @@ def _persisted_icon_response() -> Optional[Response]:
             # Check icons object in manifest
             manifest_icons = manifest.get("icons", {})
             if manifest_icons:
-                # Get the largest icon
-                largest_size = max(manifest_icons.keys(), key=lambda x: int(x))
+                # Get the largest icon - only consider numeric keys (e.g. "128", "64")
+                # Some extensions use non-numeric keys like "48x48" which would cause ValueError
+                # isinstance guard handles edge cases where manifest was parsed outside of json.load
+                numeric_keys = [k for k in manifest_icons.keys() if isinstance(k, str) and k.isdigit()]
+                if not numeric_keys:
+                    raise ValueError(f"No numeric icon size keys found in manifest: {list(manifest_icons.keys())}")
+                largest_size = max(numeric_keys, key=lambda x: int(x))
                 icon_rel_path = manifest_icons[largest_size]
                 manifest_icon_path = os.path.join(extracted_path, icon_rel_path)