diff --git a/crates/polars-ops/src/series/ops/horizontal.rs b/crates/polars-ops/src/series/ops/horizontal.rs
index 025779e7734..6a6960480c4 100644
--- a/crates/polars-ops/src/series/ops/horizontal.rs
+++ b/crates/polars-ops/src/series/ops/horizontal.rs
@@ -221,9 +221,9 @@ pub fn sum_horizontal(
 
     // If we have any null columns and null strategy is not `Ignore`, we can return immediately.
     if !ignore_nulls && non_null_cols.len() < columns.len() {
-        // We must first determine the correct return dtype.
+        // We must determine the correct return dtype.
         let return_dtype = match dtypes_to_supertype(non_null_cols.iter().map(|c| c.dtype()))? {
-            DataType::Boolean => DataType::UInt32,
+            DataType::Boolean => IDX_DTYPE,
             dt => dt,
         };
         return Ok(Some(Column::full_null(
@@ -244,7 +244,7 @@ pub fn sum_horizontal(
         },
         1 => Ok(Some(
             apply_null_strategy(if non_null_cols[0].dtype() == &DataType::Boolean {
-                non_null_cols[0].cast(&DataType::UInt32)?
+                non_null_cols[0].cast(&IDX_DTYPE)?
             } else {
                 non_null_cols[0].clone()
             })?
diff --git a/crates/polars-plan/src/dsl/function_expr/schema.rs b/crates/polars-plan/src/dsl/function_expr/schema.rs
index d45f75c01e9..beaacac4994 100644
--- a/crates/polars-plan/src/dsl/function_expr/schema.rs
+++ b/crates/polars-plan/src/dsl/function_expr/schema.rs
@@ -331,11 +331,10 @@ impl FunctionExpr {
             MinHorizontal => mapper.map_to_supertype(),
             SumHorizontal { .. } => {
                 mapper.map_to_supertype().map(|mut f| {
-                    match f.dtype {
-                        // Booleans sum to UInt32.
-                        DataType::Boolean => { f.dtype = DataType::UInt32; f},
-                        _ => f,
+                    if f.dtype == DataType::Boolean {
+                        f.dtype = IDX_DTYPE;
                     }
+                    f
                 })
             },
             MeanHorizontal { .. } => {
diff --git a/py-polars/tests/unit/operations/aggregation/test_horizontal.py b/py-polars/tests/unit/operations/aggregation/test_horizontal.py
index 3959e15e22e..bc557a231d7 100644
--- a/py-polars/tests/unit/operations/aggregation/test_horizontal.py
+++ b/py-polars/tests/unit/operations/aggregation/test_horizontal.py
@@ -319,6 +319,39 @@ def test_sum_single_col() -> None:
     )
 
 
+@pytest.mark.parametrize("ignore_nulls", [False, True])
+def test_sum_correct_supertype(ignore_nulls: bool) -> None:
+    values = [1, 2] if ignore_nulls else [None, None]  # type: ignore[list-item]
+    lf = pl.LazyFrame(
+        {
+            "null": [None, None],
+            "int": pl.Series(values, dtype=pl.Int32),
+            "float": pl.Series(values, dtype=pl.Float32),
+        }
+    )
+
+    # null + int32 should produce int32
+    out = lf.select(pl.sum_horizontal("null", "int", ignore_nulls=ignore_nulls))
+    expected = pl.LazyFrame({"null": pl.Series(values, dtype=pl.Int32)})
+    assert_frame_equal(out.collect(), expected.collect())
+    assert out.collect_schema() == expected.collect_schema()
+
+    # null + float32 should produce float32
+    out = lf.select(pl.sum_horizontal("null", "float", ignore_nulls=ignore_nulls))
+    expected = pl.LazyFrame({"null": pl.Series(values, dtype=pl.Float32)})
+    assert_frame_equal(out.collect(), expected.collect())
+    assert out.collect_schema() == expected.collect_schema()
+
+    # null + int32 + float32 should produce float64
+    values = [2, 4] if ignore_nulls else [None, None]  # type: ignore[list-item]
+    out = lf.select(
+        pl.sum_horizontal("null", "int", "float", ignore_nulls=ignore_nulls)
+    )
+    expected = pl.LazyFrame({"null": pl.Series(values, dtype=pl.Float64)})
+    assert_frame_equal(out.collect(), expected.collect())
+    assert out.collect_schema() == expected.collect_schema()
+
+
 def test_cum_sum_horizontal() -> None:
     df = pl.DataFrame(
         {
@@ -541,8 +574,8 @@ def test_horizontal_sum_boolean_with_null() -> None:
 
     expected_schema = pl.Schema(
         {
-            "null_first": pl.UInt32,
-            "bool_first": pl.UInt32,
+            "null_first": pl.get_index_type(),
+            "bool_first": pl.get_index_type(),
         }
     )
 
@@ -550,8 +583,8 @@ def test_horizontal_sum_boolean_with_null() -> None:
 
     expected_df = pl.DataFrame(
         {
-            "null_first": pl.Series([1, 0], dtype=pl.UInt32),
-            "bool_first": pl.Series([1, 0], dtype=pl.UInt32),
+            "null_first": pl.Series([1, 0], dtype=pl.get_index_type()),
+            "bool_first": pl.Series([1, 0], dtype=pl.get_index_type()),
         }
     )
 
@@ -563,7 +596,7 @@ def test_horizontal_sum_boolean_with_null() -> None:
     ("dtype_in", "dtype_out"),
     [
         (pl.Null, pl.Null),
-        (pl.Boolean, pl.UInt32),
+        (pl.Boolean, pl.get_index_type()),
         (pl.UInt8, pl.UInt8),
         (pl.Float32, pl.Float32),
         (pl.Float64, pl.Float64),
@@ -589,6 +622,7 @@ def test_horizontal_sum_with_null_col_ignore_strategy(
         values = [None, None, None]  # type: ignore[list-item]
     expected = pl.LazyFrame(pl.Series("null", values, dtype=dtype_out))
     assert_frame_equal(result, expected)
+    assert result.collect_schema() == expected.collect_schema()
 
 
 @pytest.mark.parametrize("ignore_nulls", [True, False])