diff --git a/docs/_source/defaults.rst b/docs/_source/defaults.rst
new file mode 100644
index 0000000000000000000000000000000000000000..775134f5761b27cbdc6927efaf3e3d6fa1dd68cf
--- /dev/null
+++ b/docs/_source/defaults.rst
@@ -0,0 +1,67 @@
+Defaults
+--------
+
+In this section, we explain which parameters are set by MLAir during the :py:`ExperimentSetup` if not specified by the
+user. This is important information for example if a new :ref:`Custom Data Handler` is implemented.
+
+================================= =============== ============================================================
+parameter                         default         comment
+================================= =============== ============================================================
+batch_path
+batch_size
+bootstrap_path
+competitor_path
+competitors
+create_new_bootstraps
+create_new_model
+data_handler
+data_origin
+data_path
+dimensions
+end
+epochs
+evaluate_bootstraps
+experiment_name
+experiment_path
+extreme_values
+extremes_on_right_tail_only
+forecast_path
+fraction_of_training
+hostname
+hpc_hosts
+interpolation_limit
+interpolation_method
+logging_path
+login_nodes
+model_class
+model_path
+neighbors
+number_of_bootstraps
+overwrite_local_data
+permute_data
+plot_list
+plot_path
+start
+stations
+statistics_per_var
+target_dim
+target_var
+test_start
+test_end
+test_min_length
+time_dim
+train_model
+train_end
+train_min_length
+train_start
+transformation                    :py:`{}`        implement all further transformation functionality
+                                                  inside your custom data handler
+use_all_stations_on_all_data_sets
+upsampling
+val_end
+val_min_length
+val_start
+variables
+window_history_size
+window_lead_time
+================================= =============== ============================================================
diff --git a/docs/_source/index.rst b/docs/_source/index.rst
index 705ab91ddc1000edfa7961719db1ff913de26a93..e0087b0e69fa2c63d1d976e4d0b076747e007d5f 100644
--- a/docs/_source/index.rst
+++ b/docs/_source/index.rst
@@ -1,12 +1,12 @@
 .. MLair documentation master file, created by
-    sphinx-quickstart on Wed Apr 15 14:27:29 2020.
-    You can adapt this file completely to your liking, but it should at least
-    contain the root `toctree` directive.
+sphinx-quickstart on Wed Apr 15 14:27:29 2020.
+You can adapt this file completely to your liking, but it should at least
+contain the root `toctree` directive.
 
 Welcome to MLAir's documentation!
 ================================================
 
-This is the documation of the `MLAir package <https://gitlab.version.fz-juelich.de/toar/mlair>`_.
+This is the documentation of the `MLAir package <https://gitlab.version.fz-juelich.de/toar/mlair>`_.
 
 .. toctree::
    :maxdepth: 2
@@ -14,6 +14,7 @@ This is the documation of the `MLAir package <https://gitlab.version.fz-juelich.
 
    get-started
    customise
+   defaults
    changelog
 
 
diff --git a/mlair/run_modules/experiment_setup.py b/mlair/run_modules/experiment_setup.py
index ee8506ee6c445ccf9ac93dc0498804841538311d..a27607429148fe034d9d5f2f17c3d2caf6d3a22f 100644
--- a/mlair/run_modules/experiment_setup.py
+++ b/mlair/run_modules/experiment_setup.py
@@ -294,7 +294,7 @@ class ExperimentSetup(RunEnvironment):
         self._set_param("window_history_size", window_history_size, default=DEFAULT_WINDOW_HISTORY_SIZE)
         self._set_param("overwrite_local_data", overwrite_local_data, default=DEFAULT_OVERWRITE_LOCAL_DATA,
                         scope="preprocessing")
-        self._set_param("transformation", transformation, default=None)
+        self._set_param("transformation", transformation, default={})
         self._set_param("transformation", None, scope="preprocessing")
         self._set_param("data_handler", data_handler, default=DefaultDataHandler)