diff --git a/src/ga4gh/cat_vrs/recipes.py b/src/ga4gh/cat_vrs/recipes.py
index 52374e7..bc3e62a 100644
--- a/src/ga4gh/cat_vrs/recipes.py
+++ b/src/ga4gh/cat_vrs/recipes.py
@@ -180,7 +180,8 @@ def validate_constraints(cls, v: list[Constraint]) -> list[Constraint]:
             ):
                 def_loc_constr_found = True
 
-                for r in constraint.relations:
+                relations = constraint.relations or []
+                for r in relations:
                     if r.primaryCoding and (
                         r.primaryCoding.code.root == Relation.LIFTOVER_TO.value
                         and r.primaryCoding.system
diff --git a/tests/validation/test_cat_vrs_models.py b/tests/validation/test_cat_vrs_models.py
index 87c5e4a..7b7f3e7 100644
--- a/tests/validation/test_cat_vrs_models.py
+++ b/tests/validation/test_cat_vrs_models.py
@@ -366,7 +366,11 @@ def test_canonical_allele(defining_loc_constr, members_and_name):
         recipes.CanonicalAllele(**valid_params)
 
 
-def test_categorical_cnv(members_and_name, defining_loc_constr, copy_change_constr):
+def test_categorical_cnv(
+    members_and_name: dict,
+    defining_loc_constr: models.DefiningLocationConstraint,
+    copy_change_constr: models.CopyChangeConstraint,
+):
     """Test the CategoricalCnv validator"""
     # Valid CategoricalCnv with CopyChangeConstraint
     valid_params = deepcopy(members_and_name)
@@ -384,6 +388,20 @@ def test_categorical_cnv(members_and_name, defining_loc_constr, copy_change_cons
     ]
     assert recipes.CategoricalCnv(**valid_params)
 
+    # Invalid CategoricalCnv: relations unset on DLC
+    invalid_params = deepcopy(members_and_name)
+    invalid_dlc = deepcopy(defining_loc_constr)
+    invalid_dlc.relations = None
+    invalid_params["constraints"] = [
+        models.Constraint(root=invalid_dlc),
+        models.Constraint(root=models.CopyCountConstraint(copies=[1, 2])),
+    ]
+    with pytest.raises(
+        ValueError,
+        match="`DefiningLocationConstraint` found, but must contain at least one relation where `primaryCoding.code` is 'liftover_to' and `primaryCoding.system` is 'ga4gh-gks-term:allele-relation'.",
+    ):
+        recipes.CategoricalCnv(**invalid_params)
+
     # Invalid CategoricalCnv: No DefiningLocationConstraint
     invalid_params = deepcopy(members_and_name)
     invalid_params["constraints"] = [