diff --git a/src/cal_tools/calcat_interface2.py b/src/cal_tools/calcat_interface2.py
index 445dd158182c53db8a5dacd04c48c6cb4921144e..94969690d237c379c74964858cff0dfd2d7dd9f4 100644
--- a/src/cal_tools/calcat_interface2.py
+++ b/src/cal_tools/calcat_interface2.py
@@ -448,10 +448,10 @@ class CalibrationData(Mapping):
         res = client.get("calibration_constant_versions", params)
 
         d = {}
-        pdus = set()
+        pdus = []
 
         for ccv in res:
-            pdus.add(ccv["physical_detector_unit"])
+            pdus.append(ccv["physical_detector_unit"])
             cal_type = calibration_name(ccv["calibration_constant"]["calibration_id"])
             aggr = ccv["physical_detector_unit"]["karabo_da"]
             d.setdefault(cal_type, {})[aggr] = SingleConstantVersion.from_response(ccv)