diff --git a/tests/resolver/test_lib.py b/tests/resolver/test_lib.py
index 80fe1c1..2d32449 100644
--- a/tests/resolver/test_lib.py
+++ b/tests/resolver/test_lib.py
@@ -7,7 +7,6 @@
 import pytest
 from deepdiff import DeepDiff
 from deepdiff.operator import BaseOperator
-from variantlib.errors import ConfigurationError
 from variantlib.errors import ValidationError
 from variantlib.models.variant import VariantDescription
 from variantlib.models.variant import VariantFeature
@@ -609,6 +608,7 @@ def test_sort_and_filter_supported_variants_validation_errors(
         sort_and_filter_supported_variants(
             vdescs=vdescs,
             supported_vprops=vprops,  # type: ignore[arg-type]
+            namespace_priorities=["a"],
             feature_priorities=feature_priorities,
         )
 
@@ -617,8 +617,11 @@ def test_sort_and_filter_supported_variants_validation_errors_with_no_priority(
     vdescs: list[VariantDescription], vprops: list[VariantProperty]
 ) -> None:
     # This one specifies no ordering/priority => can't sort
-    with pytest.raises(ConfigurationError, match="The variant environment needs"):
+    with pytest.raises(
+        ValidationError, match=r"Missing namespace_priorities for namespaces"
+    ):
         sort_and_filter_supported_variants(
             vdescs=vdescs,
             supported_vprops=vprops,
+            namespace_priorities=[],
         )
diff --git a/tests/resolver/test_sorting.py b/tests/resolver/test_sorting.py
index 9b17338..3966aeb 100644
--- a/tests/resolver/test_sorting.py
+++ b/tests/resolver/test_sorting.py
@@ -4,7 +4,6 @@
 from typing import Any
 
 import pytest
-from variantlib.errors import ConfigurationError
 from variantlib.errors import ValidationError
 from variantlib.models.variant import VariantDescription
 from variantlib.models.variant import VariantFeature
@@ -111,7 +110,6 @@ def test_get_namespace_priorities() -> None:
 def test_negative_get_namespace_priorities() -> None:
     vprop = VariantProperty(namespace="omnicorp", feature="no_exist", value="value")
 
-    assert get_namespace_priorities(vprop, None) == sys.maxsize
     assert get_namespace_priorities(vprop, []) == sys.maxsize
     assert get_namespace_priorities(vprop, ["other_corp"]) == sys.maxsize
 
@@ -119,13 +117,13 @@ def test_negative_get_namespace_priorities() -> None:
 @pytest.mark.parametrize(
     ("vprop", "namespace_priorities"),
     [
-        ("not a `VariantProperty`", None),
+        ("not a `VariantProperty`", []),
         (VariantProperty("a", "b", "c"), "not a list or None"),
         (VariantProperty("a", "b", "c"), [{"not a str": True}]),
     ],
 )
 def test_get_namespace_priorities_validation_error(
-    vprop: VariantProperty, namespace_priorities: list[str] | None
+    vprop: VariantProperty, namespace_priorities: list[str]
 ) -> None:
     with pytest.raises(ValidationError):
         get_namespace_priorities(vprop=vprop, namespace_priorities=namespace_priorities)
@@ -208,10 +206,10 @@ def test_sort_variant_properties_validation_error(
 
 
 def test_sort_variant_properties_configuration_error() -> None:
-    with pytest.raises(ConfigurationError):
+    with pytest.raises(ValidationError):
         sort_variant_properties(
             vprops=[VariantProperty("a", "b", "c"), VariantProperty("x", "y", "z")],
-            namespace_priorities=None,
+            namespace_priorities=[],
             feature_priorities=None,
             property_priorities=None,
         )
diff --git a/variantlib/resolver/lib.py b/variantlib/resolver/lib.py
index 7cc00a8..f81daf4 100644
--- a/variantlib/resolver/lib.py
+++ b/variantlib/resolver/lib.py
@@ -103,7 +103,7 @@ def filter_variants(
 def sort_and_filter_supported_variants(
     vdescs: list[VariantDescription],
     supported_vprops: list[VariantProperty],
-    namespace_priorities: list[VariantNamespace] | None = None,
+    namespace_priorities: list[VariantNamespace],
     feature_priorities: dict[VariantNamespace, list[VariantFeatureName]] | None = None,
     property_priorities: dict[
         VariantNamespace, dict[VariantFeatureName, list[VariantFeatureValue]]
diff --git a/variantlib/resolver/sorting.py b/variantlib/resolver/sorting.py
index ec44fbe..1de41d8 100644
--- a/variantlib/resolver/sorting.py
+++ b/variantlib/resolver/sorting.py
@@ -7,7 +7,6 @@
 from itertools import chain
 from itertools import groupby
 
-from variantlib.errors import ConfigurationError
 from variantlib.errors import ValidationError
 from variantlib.models.variant import VariantDescription
 from variantlib.models.variant import VariantProperty
@@ -79,7 +78,7 @@ def get_feature_priorities(
 
 def get_namespace_priorities(
     vprop: VariantProperty,
-    namespace_priorities: list[VariantNamespace] | None,
+    namespace_priorities: list[VariantNamespace],
 ) -> int:
     """
     Get the namespace priority of a `VariantProperty` object.
@@ -90,8 +89,6 @@ def get_namespace_priorities(
     """
     validate_type(vprop, VariantProperty)
 
-    if namespace_priorities is None:
-        return sys.maxsize
     validate_type(namespace_priorities, list[str])
 
     # if not present push at the end
@@ -103,7 +100,7 @@ def get_namespace_priorities(
 
 def sort_variant_properties(
     vprops: list[VariantProperty],
-    namespace_priorities: list[VariantNamespace] | None,
+    namespace_priorities: list[VariantNamespace],
     feature_priorities: dict[VariantNamespace, list[VariantFeatureName]] | None = None,
     property_priorities: dict[
         VariantNamespace, dict[VariantFeatureName, list[VariantFeatureValue]]
@@ -120,9 +117,7 @@ def sort_variant_properties(
     :return: Sorted list of `VariantProperty` objects.
     """
     validate_type(vprops, list[VariantProperty])
-
-    if namespace_priorities is not None:
-        validate_type(namespace_priorities, list[VariantNamespace])
+    validate_type(namespace_priorities, list[VariantNamespace])
 
     if feature_priorities is not None:
         validate_type(
@@ -134,22 +129,10 @@ def sort_variant_properties(
             dict[VariantNamespace, dict[VariantFeatureName, list[VariantFeatureValue]]],
         )
 
-    error_message = (
-        "The variant environment needs to be (re)configured, please execute "
-        "`variantlib config setup` and re-run your command."
-    )
-
     found_namespaces = {vprop.namespace for vprop in vprops}
 
-    if namespace_priorities is None or not namespace_priorities:
-        if len(found_namespaces) > 1:
-            raise ConfigurationError(error_message)
-
-        # if there is only one namespace, use it as the default
-        namespace_priorities = list(found_namespaces)
-
-    elif len(found_namespaces.difference(namespace_priorities)) > 0:
-        raise ConfigurationError(error_message)
+    if missing := found_namespaces.difference(namespace_priorities):
+        raise ValidationError(f"Missing namespace_priorities for namespaces {missing}")
 
     # 1. Reorder properties according to namespace priorities.
     sorted_by_namespace = sorted(