diff --git a/fluents/dataset.py b/fluents/dataset.py
index d1db1d1..b7fde34 100644
--- a/fluents/dataset.py
+++ b/fluents/dataset.py
@@ -183,6 +183,8 @@ class Dataset:
         matrix in a given dim.  If none of the input identifiers are
         found an empty index is returned
         """
+        if not isinstance(idents, list) and not isinstance(idents, set):
+            raise ValueError("idents needs to be a list/set got: %s" %type(idents))
         if idents==None:
             index = array_sort(self._map[dim].values())
         else: