diff --git a/janus_core/helpers/utils.py b/janus_core/helpers/utils.py
index fa1933d9..fb3d4939 100644
--- a/janus_core/helpers/utils.py
+++ b/janus_core/helpers/utils.py
@@ -428,7 +428,7 @@ def check_files_exist(config: dict, req_file_keys: Sequence[PathLike]) -> None:
         If a key from `req_file_keys` is in the configuration file, but the
         file corresponding to the configuration value do not exist.
     """
-    for file_key in req_file_keys:
+    for file_key in config.keys() & req_file_keys:
         # Only check if file key is in the configuration file
-        if file_key in config and not Path(config[file_key]).exists():
+        if not Path(config[file_key]).exists():
             raise FileNotFoundError(f"{config[file_key]} does not exist")
diff --git a/janus_core/training/preprocess.py b/janus_core/training/preprocess.py
index dcfc0be0..857042a9 100644
--- a/janus_core/training/preprocess.py
+++ b/janus_core/training/preprocess.py
@@ -16,7 +16,7 @@
 
 def preprocess(
     mlip_config: PathLike,
-    req_file_keys: Sequence[PathLike] | None = None,
+    req_file_keys: Sequence[PathLike] = ("train_file", "test_file", "valid_file"),
     attach_logger: bool = False,
     log_kwargs: dict[str, Any] | None = None,
     track_carbon: bool = True,
@@ -32,7 +32,7 @@ def preprocess(
     ----------
     mlip_config : PathLike
         Configuration file to pass to MLIP.
-    req_file_keys : Sequence[PathLike] | None
+    req_file_keys : Sequence[PathLike]
         List of files that must exist if defined in the configuration file.
         Default is ("train_file", "test_file", "valid_file").
     attach_logger : bool
@@ -44,10 +44,7 @@ def preprocess(
     tracker_kwargs : dict[str, Any] | None
         Keyword arguments to pass to `config_tracker`. Default is {}.
     """
-    (log_kwargs, tracker_kwargs) = none_to_dict((log_kwargs, tracker_kwargs))
-
-    if req_file_keys is None:
-        req_file_keys = ("train_file", "test_file", "valid_file")
+    log_kwargs, tracker_kwargs = none_to_dict((log_kwargs, tracker_kwargs))
 
     # Validate inputs
     with open(mlip_config, encoding="utf8") as file:
diff --git a/janus_core/training/train.py b/janus_core/training/train.py
index 710c0a47..8cf0f44b 100644
--- a/janus_core/training/train.py
+++ b/janus_core/training/train.py
@@ -16,7 +16,12 @@
 
 def train(
     mlip_config: PathLike,
-    req_file_keys: Sequence[PathLike] | None = None,
+    req_file_keys: Sequence[PathLike] = (
+        "train_file",
+        "test_file",
+        "valid_file",
+        "statistics_file",
+    ),
     attach_logger: bool = False,
     log_kwargs: dict[str, Any] | None = None,
     track_carbon: bool = True,
@@ -32,7 +37,7 @@ def train(
     ----------
     mlip_config : PathLike
         Configuration file to pass to MLIP.
-    req_file_keys : Sequence[PathLike] | None
+    req_file_keys : Sequence[PathLike]
         List of files that must exist if defined in the configuration file.
         Default is ("train_file", "test_file", "valid_file", "statistics_file").
     attach_logger : bool
@@ -44,10 +49,7 @@ def train(
     tracker_kwargs : Optional[dict[str, Any]]
         Keyword arguments to pass to `config_tracker`. Default is {}.
     """
-    (log_kwargs, tracker_kwargs) = none_to_dict((log_kwargs, tracker_kwargs))
-
-    if req_file_keys is None:
-        req_file_keys = ("train_file", "test_file", "valid_file", "statistics_file")
+    log_kwargs, tracker_kwargs = none_to_dict((log_kwargs, tracker_kwargs))
 
     # Validate inputs
     with open(mlip_config, encoding="utf8") as file: