diff --git a/csaxs_bec/bec_widgets/widgets/sample_storage/sample_storage.py b/csaxs_bec/bec_widgets/widgets/sample_storage/sample_storage.py
index 1735c3d..e14d74c 100644
--- a/csaxs_bec/bec_widgets/widgets/sample_storage/sample_storage.py
+++ b/csaxs_bec/bec_widgets/widgets/sample_storage/sample_storage.py
@@ -428,10 +428,17 @@ class SampleStorageWidget(BECWidget, QWidget):
         owner, ok = QInputDialog.getText(
             self,
             "Sample owner",
-            f"Owner for slot {self._slot_title(slot)} (optional):",
+            f"Owner for slot {self._slot_title(slot)}:",
             text=current_owner,
         )
-        owner = owner.strip() if ok else current_owner
+        if not ok:
+            return
+        owner = owner.strip()
+        if not owner:
+            QMessageBox.warning(
+                self, "Invalid owner", "An occupied slot needs a non-empty owner."
+            )
+            return
         if self._write_slot(slot, 1, name, owner):
             self.refresh()
 
@@ -447,9 +454,16 @@ class SampleStorageWidget(BECWidget, QWidget):
             QMessageBox.warning(self, "Invalid name", "Please enter a non-empty sample name.")
             return
         owner, ok = QInputDialog.getText(
-            self, "Sample owner", f"Owner for slot {self._slot_title(slot)} (optional):"
+            self, "Sample owner", f"Owner for slot {self._slot_title(slot)}:"
         )
-        owner = owner.strip() if ok else ""
+        if not ok:
+            return
+        owner = owner.strip()
+        if not owner:
+            QMessageBox.warning(
+                self, "Invalid owner", "An occupied slot needs a non-empty owner."
+            )
+            return
         if self._write_slot(slot, 1, name, owner):
             self.refresh()