diff --git a/pint/delegates/formatter/_compound_unit_helpers.py b/pint/delegates/formatter/_compound_unit_helpers.py
index c8ac1e2d9..46934c063 100644
--- a/pint/delegates/formatter/_compound_unit_helpers.py
+++ b/pint/delegates/formatter/_compound_unit_helpers.py
@@ -246,6 +246,7 @@ def prepare_compount_unit(
     locale: Locale | str | None = None,
     as_ratio: bool = True,
     registry: UnitRegistry | None = None,
+    empty_numerator_fmt="1",
 ) -> tuple[Iterable[tuple[str, T]], Iterable[tuple[str, T]]]:
     """Format compound unit into unit container given
     an spec and locale.
diff --git a/pint/delegates/formatter/plain.py b/pint/delegates/formatter/plain.py
index d03c00959..5cf77f3e0 100644
--- a/pint/delegates/formatter/plain.py
+++ b/pint/delegates/formatter/plain.py
@@ -81,7 +81,7 @@ def format_unit(
         unit: PlainUnit | Iterable[tuple[str, Any]],
         uspec: str = "",
         sort_func: SortFunc | None = None,
-        empty_numerator_fmt = "1",
+        empty_numerator_fmt="1",
         **babel_kwds: Unpack[BabelKwds],
     ) -> str:
         """Format a unit (can be compound) into string
@@ -204,7 +204,7 @@ def format_unit(
         unit: PlainUnit | Iterable[tuple[str, Any]],
         uspec: str = "",
         sort_func: SortFunc | None = None,
-        empty_numerator_fmt = "1",
+        empty_numerator_fmt="1",
         **babel_kwds: Unpack[BabelKwds],
     ) -> str:
         numerator, denominator = prepare_compount_unit(
@@ -316,16 +316,13 @@ def format_unit(
         unit: PlainUnit | Iterable[tuple[str, Any]],
         uspec: str = "",
         sort_func: SortFunc | None = None,
-                                        empty_numerator_fmt="1",
-
+        empty_numerator_fmt="1",
         **babel_kwds: Unpack[BabelKwds],
-
     ) -> str:
         numerator, denominator = prepare_compount_unit(
             unit,
             uspec,
             sort_func=sort_func,
-
             **babel_kwds,
             registry=self._registry,
         )
@@ -346,8 +343,7 @@ def format_unit(
             power_fmt="{}{}",
             parentheses_fmt="({})",
             exp_call=pretty_fmt_exponent,
-                        empty_numerator_fmt=empty_numerator_fmt,
-
+            empty_numerator_fmt=empty_numerator_fmt,
         )
 
     def format_quantity(