diff --git a/narwhals/_arrow/dataframe.py b/narwhals/_arrow/dataframe.py
index 16669719e..8576dd2b2 100644
--- a/narwhals/_arrow/dataframe.py
+++ b/narwhals/_arrow/dataframe.py
@@ -641,7 +641,7 @@ def is_unique(self: Self) -> ArrowSeries:
 
     def unique(
         self: Self,
-        subset: str | list[str] | None,
+        subset: list[str] | None,
         *,
         keep: Literal["any", "first", "last", "none"],
         maintain_order: bool,
@@ -653,9 +653,6 @@ def unique(
         import pyarrow.compute as pc
 
         df = self._native_frame
-
-        if isinstance(subset, str):
-            subset = [subset]
         subset = subset or self.columns
 
         if keep in {"any", "first", "last"}:
diff --git a/narwhals/_dask/dataframe.py b/narwhals/_dask/dataframe.py
index 0e762b1fa..0edf41216 100644
--- a/narwhals/_dask/dataframe.py
+++ b/narwhals/_dask/dataframe.py
@@ -198,14 +198,13 @@ def head(self: Self, n: int) -> Self:
 
     def unique(
         self: Self,
-        subset: str | list[str] | None,
+        subset: list[str] | None,
         *,
         keep: Literal["any", "first", "last", "none"] = "any",
         maintain_order: bool = False,
     ) -> Self:
         # The param `maintain_order` is only here for compatibility with the Polars API
         # and has no effect on the output.
-        subset = flatten(subset) if subset else None
         native_frame = self._native_frame
         if keep == "none":
             subset = subset or self.columns
diff --git a/narwhals/_pandas_like/dataframe.py b/narwhals/_pandas_like/dataframe.py
index 6f8706ae4..c10aacec5 100644
--- a/narwhals/_pandas_like/dataframe.py
+++ b/narwhals/_pandas_like/dataframe.py
@@ -684,7 +684,7 @@ def tail(self, n: int) -> Self:
 
     def unique(
         self: Self,
-        subset: str | list[str] | None,
+        subset: list[str] | None,
         *,
         keep: Literal["any", "first", "last", "none"] = "any",
         maintain_order: bool = False,
@@ -692,7 +692,6 @@ def unique(
         # The param `maintain_order` is only here for compatibility with the Polars API
         # and has no effect on the output.
         mapped_keep = {"none": False, "any": "first"}.get(keep, keep)
-        subset = flatten(subset) if subset else None
         return self._from_native_frame(
             self._native_frame.drop_duplicates(subset=subset, keep=mapped_keep)
         )
diff --git a/narwhals/dataframe.py b/narwhals/dataframe.py
index 2b59dd952..ccdba365e 100644
--- a/narwhals/dataframe.py
+++ b/narwhals/dataframe.py
@@ -153,6 +153,8 @@ def unique(
         keep: Literal["any", "first", "last", "none"] = "any",
         maintain_order: bool = False,
     ) -> Self:
+        if isinstance(subset, str):
+            subset = [subset]
         return self._from_compliant_dataframe(
             self._compliant_frame.unique(
                 subset=subset, keep=keep, maintain_order=maintain_order