diff --git a/pint/facets/plain/registry.py b/pint/facets/plain/registry.py
index 903620b2e..903fad9d1 100644
--- a/pint/facets/plain/registry.py
+++ b/pint/facets/plain/registry.py
@@ -1167,8 +1167,8 @@ def parse_units(
     def _parse_units(
         self,
         input_string: str,
-        as_delta: bool = True,
-        case_sensitive: Optional[bool] = None,
+        as_delta: bool,
+        case_sensitive: bool,
     ) -> UnitsContainer:
         """Parse a units expression and returns a UnitContainer with
         the canonical names.
diff --git a/pint/util.py b/pint/util.py
index e940ea6c2..1f7c29161 100644
--- a/pint/util.py
+++ b/pint/util.py
@@ -1039,8 +1039,7 @@ def to_units_container(
         return unit_like._units
     elif str in mro:
         if registry:
-            # TODO: Why not parse.units here?
-            return registry._parse_units(unit_like)
+            return registry.parse_units(unit_like)._units
         else:
             return ParserHelper.from_string(unit_like)
     elif dict in mro: