diff --git a/src/postprocessor/grouper.py b/src/postprocessor/grouper.py
index 0dca70e0e31f56117077cec0d1cf422956a9bb5c..ac828589f6411ffa61c6b2e4c5965a31091b246d 100644
--- a/src/postprocessor/grouper.py
+++ b/src/postprocessor/grouper.py
@@ -289,15 +289,17 @@ def concat_one_signal(
         combined = position.get_raw(path, **kwargs)
     elif mode == "daughters":
         combined = position.get_raw(path, lineage=True, **kwargs)
-        combined = combined.loc[
-            combined.index.get_level_values("mother_label") > 0
-        ]
+        if combined is not None:
+            combined = combined.loc[
+                combined.index.get_level_values("mother_label") > 0
+            ]
     elif mode == "mothers":
         combined = position.get_raw(path, lineage=True, **kwargs)
-        combined = combined.loc[
-            combined.index.get_level_values("mother_label") == 0
-        ]
-        combined = combined.droplevel("mother_label")
+        if combined is not None:
+            combined = combined.loc[
+                combined.index.get_level_values("mother_label") == 0
+            ]
+            combined = combined.droplevel("mother_label")
     elif mode == "families":
         combined = position[path]
     else: