diff --git a/pyerrors/pyerrors.py b/pyerrors/pyerrors.py
index e7ecfe8b..81ee81f9 100644
--- a/pyerrors/pyerrors.py
+++ b/pyerrors/pyerrors.py
@@ -1162,6 +1162,8 @@ def correlate(obs_a, obs_b):
     for name in obs_a.names:
         if obs_a.shape[name] != obs_b.shape[name]:
             raise Exception('Shapes of ensemble', name, 'do not fit')
+        if obs_a.idl[name] != obs_b.idl[name]:
+            raise Exception('idl of ensemble', name, 'do not fit')
 
     if obs_a.reweighted is True:
         warnings.warn("The first observable is already reweighted.", RuntimeWarning)