diff --git a/src/_gettsim/interface.py b/src/_gettsim/interface.py
index 37e56634c..d9da8188f 100644
--- a/src/_gettsim/interface.py
+++ b/src/_gettsim/interface.py
@@ -8,7 +8,13 @@
 import dags
 import networkx
 import pandas as pd
-from optree import tree_flatten, tree_flatten_with_path, tree_map, tree_map_with_path
+from optree import (
+    tree_flatten,
+    tree_flatten_with_path,
+    tree_map,
+    tree_map_with_path,
+    tree_unflatten,
+)
 
 from _gettsim.config import (
     DEFAULT_TARGETS,
@@ -841,8 +847,9 @@ def _round_and_partial_parameters_to_functions(
     # Partial parameters to functions such that they disappear in the DAG.
     # Note: Needs to be done after rounding such that dags recognizes partialled
     # parameters.
-    processed_functions = {}
-    for name, function in functions.items():
+    function_leafs, tree_spec = tree_flatten(functions)
+    processed_functions = []
+    for function in function_leafs:
         arguments = get_names_of_arguments_without_defaults(function)
         partial_params = {
             i: params[i[:-7]]
@@ -857,11 +864,11 @@ def _round_and_partial_parameters_to_functions(
             if hasattr(function, "__info__"):
                 partial_func.__info__ = function.__info__
 
-            processed_functions[name] = partial_func
+            processed_functions.append(partial_func)
         else:
-            processed_functions[name] = function
+            processed_functions.append(function)
 
-    return processed_functions
+    return tree_unflatten(tree_spec, processed_functions)
 
 
 def _add_rounding_to_function(