diff --git a/baybe/acquisition/base.py b/baybe/acquisition/base.py
index 41c80bf246..173b870e64 100644
--- a/baybe/acquisition/base.py
+++ b/baybe/acquisition/base.py
@@ -5,7 +5,7 @@
 import warnings
 from abc import ABC
 from inspect import signature
-from typing import ClassVar, Union
+from typing import ClassVar, Optional, Union
 
 import pandas as pd
 from attrs import define
@@ -36,8 +36,8 @@ def is_mc(cls) -> bool:
     def to_botorch(
         self,
         surrogate: Surrogate,
-        train_x: pd.DataFrame,
-        train_y: pd.DataFrame,
+        train_x: Optional[pd.DataFrame],
+        train_y: Optional[pd.DataFrame],
     ):
         """Create the botorch-ready representation of the function."""
         import botorch.acquisition as botorch_analytical_acqf