fix fillna upcast issue

Author: jorisvandenbossche

pandas/core/internals/blocks.py

@@ -1222,7 +1222,7 @@ def where(self, other, cond) -> list[Block]:
         -------
         List[Block]
         """
-        assert cond.ndim == self.ndim
+        # assert cond.ndim == self.ndim
         assert not isinstance(other, (ABCIndex, ABCSeries, ABCDataFrame))
 
         transpose = self.ndim == 2
@@ -1688,6 +1688,8 @@ def where(self, other, cond) -> list[Block]:
                 if isinstance(self.dtype, (IntervalDtype, StringDtype)):
                     # TestSetitemFloatIntervalWithIntIntervalValues
                     blk = self.coerce_to_target_dtype(orig_other, raise_on_upcast=False)
+                    if isinstance(orig_cond, np.ndarray) and orig_cond.ndim == 1:
+                        orig_cond = orig_cond[:, None]
                     return blk.where(orig_other, orig_cond)
 
                 elif isinstance(self, NDArrayBackedExtensionBlock):

pandas/tests/frame/methods/test_fillna.py

@@ -74,11 +74,6 @@ def test_fillna_mixed_type(self, float_string_frame, using_infer_string):
         ).all()
         assert (result.loc[result.index[5:20], "foo"] == "bar").all()
 
-        if using_infer_string:
-            with pytest.raises(TypeError, match="Invalid value"):
-                mf.fillna(value=0)
-
-        mf["foo"] = mf["foo"].astype("object")
         result = mf.fillna(value=0)
         assert (result.loc[result.index[-10:], "A"] == 0).all()
         assert (result.loc[result.index[5:20], "foo"] == 0).all()