diff --git a/src/cal_tools/calcat_interface2.py b/src/cal_tools/calcat_interface2.py
index a69ea576dd6d63340e17c7f61f122e44a35d3de6..5207f5e7245d13781f9d5980c7f5dd6e0983f084 100644
--- a/src/cal_tools/calcat_interface2.py
+++ b/src/cal_tools/calcat_interface2.py
@@ -340,7 +340,7 @@ class MultiModuleConstant(Mapping):
         for m in self.module_details:
             names = (m["module_number"], m["virtual_device_name"], m["physical_name"])
             if key in names and m["karabo_da"] in self.constants:
-                candidate_kdas.add([m["karabo_da"]])
+                candidate_kdas.add(m["karabo_da"])
 
         if not candidate_kdas:
             raise KeyError(key)