pandas-devGH-24196: regression test on sum nan implemented

Author: gmaiwald

pandas/tests/groupby/test_groupby.py

@@ -2970,3 +2970,16 @@ def test_groupby_numeric_only_std_no_result(numeric_only):
             ValueError, match="could not convert string to float: 'bar'"
         ):
             dfgb.std(numeric_only=numeric_only)
+
+
+@pytest.mark.parametrize("bug_var", [1, "a"])
+def test_groupby_sum_on_nan_should_return_nan(bug_var):
+    # GH 24196
+    df = pd.DataFrame({
+        'A': [bug_var, bug_var, bug_var, np.nan]
+    })
+    dfgb = df.groupby(lambda x: x)
+    result = dfgb.sum(min_count=1)
+
+    expected_df = DataFrame([bug_var, bug_var, bug_var, np.nan], columns=["A"])
+    tm.assert_frame_equal(result, expected_df)