diff --git a/quickstats/components/workspaces/xml_ws_builder_v2.py b/quickstats/components/workspaces/xml_ws_builder_v2.py
index b69c649807977d6dbb2fcec9a5d80459fb2ab8f4..4076d8ac6f3e86ddfef05fef3a9ca2a691b86f43 100644
--- a/quickstats/components/workspaces/xml_ws_builder_v2.py
+++ b/quickstats/components/workspaces/xml_ws_builder_v2.py
@@ -488,8 +488,8 @@ class XMLWSBuilder(XMLWSBase):
                 uncert_lo_expr = syst.get_uncert_lo_expr()
                 uncert_hi_name = self.import_expression(ws, uncert_hi_expr)
                 uncert_lo_name = self.import_expression(ws, uncert_lo_expr)
-                uncert_hi = ws.var(uncert_hi_name)
-                uncert_lo = ws.var(uncert_lo_name)
+                uncert_hi = ws.obj(uncert_hi_name)
+                uncert_lo = ws.obj(uncert_lo_name)
                 syst_domain.add_item(nuis, syst.nominal, uncert_hi, uncert_lo,
                                      syst.get_interp_code(), syst.constr_term)
             self.build_response_function(ws, syst_domain)