diff --git a/module_build_service/builder/KojiModuleBuilder.py b/module_build_service/builder/KojiModuleBuilder.py
index 534f2e4a..15a85961 100644
--- a/module_build_service/builder/KojiModuleBuilder.py
+++ b/module_build_service/builder/KojiModuleBuilder.py
@@ -1338,6 +1338,9 @@ class KojiModuleBuilder(GenericBuilder):
             the module build in the build system.
         :return: list of architectures
         """
+        if not module.koji_tag:
+            log.warning("No Koji tag associated with module %r", module)
+            return []
         koji_session = KojiModuleBuilder.get_session(conf, login=False)
         tag = koji_session.getTag(module.koji_tag)
         if not tag:
diff --git a/tests/test_builder/test_koji.py b/tests/test_builder/test_koji.py
index 0f990ef2..ce8b3443 100644
--- a/tests/test_builder/test_koji.py
+++ b/tests/test_builder/test_koji.py
@@ -934,6 +934,35 @@ class TestKojiBuilder:
         ret = KojiModuleBuilder.get_module_build_arches(module_build)
         assert " ".join(ret) == arches
 
+    @patch.dict("sys.modules", krbV=MagicMock())
+    @patch("module_build_service.builder.KojiModuleBuilder.KojiClientSession")
+    def test_get_module_build_arches_with_archless_tag(self, ClientSession):
+        module_build = module_build_service.models.ModuleBuild.get_by_id(db_session, 2)
+        session = ClientSession.return_value
+        session.getTag.return_value = {"arches": ""}
+        ret = KojiModuleBuilder.get_module_build_arches(module_build)
+        assert ret == []
+
+    @patch.dict("sys.modules", krbV=MagicMock())
+    @patch("module_build_service.builder.KojiModuleBuilder.KojiClientSession")
+    def test_get_module_build_arches_without_tag(self, ClientSession):
+        module_build = module_build_service.models.ModuleBuild.get_by_id(db_session, 2)
+        module_build.koji_tag = None
+        session = ClientSession.return_value
+        ret = KojiModuleBuilder.get_module_build_arches(module_build)
+        assert ret == []
+        session.getTag.assert_not_called()
+        session.assert_not_called()
+
+    @patch.dict("sys.modules", krbV=MagicMock())
+    @patch("module_build_service.builder.KojiModuleBuilder.KojiClientSession")
+    def test_get_module_build_arches_with_unknown_tag(self, ClientSession):
+        module_build = module_build_service.models.ModuleBuild.get_by_id(db_session, 2)
+        session = ClientSession.return_value
+        session.getTag.return_value = None
+        with pytest.raises(ValueError, match="Unknown Koji tag .*"):
+            KojiModuleBuilder.get_module_build_arches(module_build)
+
 
 class TestGetDistTagSRPM:
     """Test KojiModuleBuilder.get_disttag_srpm"""