DataFrame transform with fillna test GH#26840 (#52230)

Signed-off-by: Liang Yan <[email protected]>

Author: liang3zy22

pandas/tests/groupby/transform/test_transform.py

@@ -361,6 +361,20 @@ def test_dispatch_transform(tsframe):
     tm.assert_frame_equal(filled, expected)
 
 
+def test_transform_fillna_null():
+    df = DataFrame(
+        dict(
+            price=[10, 10, 20, 20, 30, 30],
+            color=[10, 10, 20, 20, 30, 30],
+            cost=(100, 200, 300, 400, 500, 600),
+        )
+    )
+    with pytest.raises(ValueError, match="Must specify a fill 'value' or 'method'"):
+        df.groupby(["price"]).transform("fillna")
+    with pytest.raises(ValueError, match="Must specify a fill 'value' or 'method'"):
+        df.groupby(["price"]).fillna()
+
+
 def test_transform_transformation_func(transformation_func):
     # GH 30918
     df = DataFrame(