diff --git a/fsl/utils/filetree/filetree.py b/fsl/utils/filetree/filetree.py
index 10afe1e2a344382f801c9e8251f38e3532b320a3..15bd8bb8841d49900362be6f2cd53860f05f782b 100644
--- a/fsl/utils/filetree/filetree.py
+++ b/fsl/utils/filetree/filetree.py
@@ -49,7 +49,7 @@ class FileTree(object):
         All tree variables including those from the parent tree
         """
         if self.parent is None:
-            return self.variables
+            return dict(self.variables)
         res = self.parent.all_variables
         res.update(self.variables)
         return res