diff --git a/beets/plugins.py b/beets/plugins.py
index 397c33822..65c181388 100644
--- a/beets/plugins.py
+++ b/beets/plugins.py
@@ -633,14 +633,16 @@ def send(event: EventType, **arguments: Any) -> list[Any]:
 
 
 def feat_tokens(
-    for_artist: bool = True, custom_feat_words: list[str] = []
+    for_artist: bool = True, custom_feat_words: list[str] | None = None
 ) -> str:
     """Return a regular expression that matches phrases like "featuring"
     that separate a main artist or a song title from secondary artists.
     The `for_artist` option determines whether the regex should be
     suitable for matching artist fields (the default) or title fields.
     """
-    feat_words = ["ft", "featuring", "feat", "feat.", "ft."] + custom_feat_words
+    feat_words = ["ft", "featuring", "feat", "feat.", "ft."]
+    if isinstance(custom_feat_words, list):
+        feat_words += custom_feat_words
     if for_artist:
         feat_words += ["with", "vs", "and", "con", "&"]
     return (
diff --git a/beetsplug/ftintitle.py b/beetsplug/ftintitle.py
index b1331e893..6837732b2 100644
--- a/beetsplug/ftintitle.py
+++ b/beetsplug/ftintitle.py
@@ -27,7 +27,9 @@ if TYPE_CHECKING:
 
 
 def split_on_feat(
-    artist: str, for_artist: bool = True, custom_feat_words: list[str] = []
+    artist: str,
+    for_artist: bool = True,
+    custom_feat_words: list[str] | None = None,
 ) -> tuple[str, str | None]:
     """Given an artist string, split the "main" artist from any artist
     on the right-hand side of a string like "feat". Return the main
@@ -46,7 +48,9 @@ def split_on_feat(
         return parts
 
 
-def contains_feat(title: str, custom_feat_words: list[str] = []) -> bool:
+def contains_feat(
+    title: str, custom_feat_words: list[str] | None = None
+) -> bool:
     """Determine whether the title contains a "featured" marker."""
     return bool(
         re.search(
@@ -60,7 +64,9 @@ def contains_feat(title: str, custom_feat_words: list[str] = []) -> bool:
 
 
 def find_feat_part(
-    artist: str, albumartist: str | None, custom_feat_words: list[str] = []
+    artist: str,
+    albumartist: str | None,
+    custom_feat_words: list[str] | None = None,
 ) -> str | None:
     """Attempt to find featured artists in the item's artist fields and
     return the results. Returns None if no featured artist found.