diff --git a/pyerrors/obs.py b/pyerrors/obs.py
index 085c9a95..7e98ae8c 100644
--- a/pyerrors/obs.py
+++ b/pyerrors/obs.py
@@ -1307,7 +1307,7 @@ def covariance2(obs1, obs2, correlation=False, **kwargs):
             if r_name not in obs2.e_content[e_name]:
                 continue
             idl_d[r_name] = _merge_idx([obs1.idl[r_name], obs2.idl[r_name]])
-            if idl_d[r_name] is range:
+            if isinstance(idl_d[r_name], range):
                 r_length.append(len(idl_d[r_name]))
             else:
                 r_length.append((idl_d[r_name][-1] - idl_d[r_name][0] + 1))