diff --git a/idm.py b/idm.py
index 61ec483..12f715f 100644
--- a/idm.py
+++ b/idm.py
@@ -469,7 +469,9 @@ class IDMProbe:
         if axis_twist_compensation is not None:
             z_compensation = (
                 axis_twist_compensation.get_z_compensation_value(pos))
-        sample["dist"] = self.freq_to_dist(sample["freq"], sample["temp"])+z_compensation
+        sample["dist"] = self.freq_to_dist(sample["freq"], sample["temp"])
+        if sample["dist"]~=None:
+            sample["dist"]=sample["dist"]+z_compensation
         sample["pos"] = pos
         sample["vel"] = vel