diff --git a/linopy/expressions.py b/linopy/expressions.py
index ca491c3e..33f6d774 100644
--- a/linopy/expressions.py
+++ b/linopy/expressions.py
@@ -284,7 +284,7 @@ def sum(self, use_fallback: bool = False, **kwargs: Any) -> LinearExpression:
                 index.names = [str(col) for col in orig_group.columns]
                 index.name = GROUP_DIM
                 new_coords = Coordinates.from_pandas_multiindex(index, GROUP_DIM)
-                ds = xr.Dataset(ds.assign_coords(new_coords))
+                ds = ds.assign_coords(new_coords)
 
             ds = ds.rename({GROUP_DIM: final_group_name})
             return LinearExpression(ds, self.model)
@@ -392,7 +392,7 @@ def __init__(self, data: Dataset | Any | None, model: Model) -> None:
         data = assign_multiindex_safe(data, **coeffs_vars_dict)
 
         # transpose with new Dataset to really ensure correct order
-        data = Dataset(data.transpose(..., TERM_DIM))
+        data = data.transpose(..., TERM_DIM)
 
         # ensure helper dimensions are not set as coordinates
         if drop_dims := set(HELPER_DIMS).intersection(data.coords):
@@ -2098,7 +2098,7 @@ def __init__(self, data: Dataset | None, model: Model) -> None:
             raise ValueError(f"Size of dimension {FACTOR_DIM} must be 2.")
 
         # transpose data to have _term as last dimension and _factor as second last
-        data = xr.Dataset(data.transpose(..., FACTOR_DIM, TERM_DIM))
+        data = data.transpose(..., FACTOR_DIM, TERM_DIM)
         self._data = data
 
     @property
@@ -2340,7 +2340,7 @@ def merge(
         LinearExpression | QuadraticExpression | variables.Variable | Dataset
     ],
     dim: str = TERM_DIM,
-    cls: type[GenericExpression] = None,  # type: ignore
+    cls: type[GenericExpression] | None = None,
     join: str | None = None,
     **kwargs: Any,
 ) -> GenericExpression:
@@ -2384,7 +2384,7 @@ def merge(
     has_quad_expression = any(type(e) is QuadraticExpression for e in exprs)
     has_linear_expression = any(type(e) is LinearExpression for e in exprs)
     if cls is None:
-        cls = QuadraticExpression if has_quad_expression else LinearExpression
+        cls = QuadraticExpression if has_quad_expression else LinearExpression  # type: ignore[assignment]
 
     if cls is QuadraticExpression and dim == TERM_DIM and has_linear_expression:
         raise ValueError(
@@ -2445,6 +2445,7 @@ def merge(
     for d in set(HELPER_DIMS) & set(ds.coords):
         ds = ds.reset_index(d, drop=True)
 
+    assert cls is not None
     return cls(ds, model)
 
 
diff --git a/linopy/model.py b/linopy/model.py
index 2a635680..57acecc2 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -359,7 +359,7 @@ def parameters(self, value: Dataset | Mapping) -> None:
         """
         Set the parameters of the model.
         """
-        self._parameters = Dataset(value)
+        self._parameters = value if isinstance(value, Dataset) else Dataset(value)
 
     @property
     def solution(self) -> Dataset: