diff --git a/datastock/_class1_interpolate.py b/datastock/_class1_interpolate.py
index d9236dc..6caf995 100644
--- a/datastock/_class1_interpolate.py
+++ b/datastock/_class1_interpolate.py
@@ -1574,7 +1574,8 @@ def _xunique(dout=None, domain=None):
     }
 
     # Number of Nones expected
-    nNone = 1 + len(domain)
+    ndom = 0 if domain is None else len(domain)
+    nNone = 1 + ndom
 
     # check
     dwrong = {k0: v0 for k0, v0 in dind.items() if len(v0) != nNone}
@@ -1583,7 +1584,7 @@ def _xunique(dout=None, domain=None):
             f"\t- {k0}: {dout[k0]['ref']} => {v0}" for k0, v0 in dwrong.items()
         ]
         msg = (
-            "Interpolate unique pt => ref should have nNone = 1 + {len(domain)}:\n"
+            "Interpolate unique pt => ref should have nNone = 1 + {ndom}:\n"
             + "\n".join(lstr)
         )
         raise Exception(msg)