diff --git a/PyExpUtils/collection/Collector.py b/PyExpUtils/collection/Collector.py
index 510d336..e124b30 100644
--- a/PyExpUtils/collection/Collector.py
+++ b/PyExpUtils/collection/Collector.py
@@ -64,6 +64,10 @@ def __init__(self, config: Dict[str, Sampler | Ignore] = {}, idx: int | None = N
         self._idxs = set[int]()
         self._keys = set[str]()
 
+        # if an idx is provided add it to the cache
+        if idx is not None:
+            self._idxs.add(idx)
+
     # -------------
     # -- Context --
     # -------------