diff --git a/crates/polars-lazy/src/physical_plan/expressions/aggregation.rs b/crates/polars-lazy/src/physical_plan/expressions/aggregation.rs
index 5393d2a6a4a03..03894f1c43c22 100644
--- a/crates/polars-lazy/src/physical_plan/expressions/aggregation.rs
+++ b/crates/polars-lazy/src/physical_plan/expressions/aggregation.rs
@@ -174,35 +174,46 @@ impl PhysicalExpr for AggregationExpr {
                                     GroupsProxy::Idx(idx) => {
                                         let s = s.rechunk();
                                         let array = &s.chunks()[0];
-                                        let validity = array.validity().unwrap();
-
-                                        let out: IdxCa = idx
-                                            .iter()
-                                            .map(|(_, g)| {
-                                                let mut count = 0 as IdxSize;
-                                                // Count valid values
-                                                g.iter().for_each(|i| {
-                                                    count += validity.get_bit_unchecked(*i as usize)
-                                                        as IdxSize;
-                                                });
-                                                count
-                                            })
-                                            .collect_ca_trusted_with_dtype(&keep_name, IDX_DTYPE);
+                                        let out: IdxCa = if matches!(s.dtype(), &DataType::Null) {
+                                            IdxCa::full(s.name(), 0, idx.first().len())
+                                        } else {
+                                            let validity = array.validity().unwrap();
+                                            idx.iter()
+                                                .map(|(_, g)| {
+                                                    let mut count = 0 as IdxSize;
+                                                    // Count valid values
+                                                    g.iter().for_each(|i| {
+                                                        count += validity
+                                                            .get_bit_unchecked(*i as usize)
+                                                            as IdxSize;
+                                                    });
+                                                    count
+                                                })
+                                                .collect_ca_trusted_with_dtype(
+                                                    &keep_name, IDX_DTYPE,
+                                                )
+                                        };
                                         AggregatedScalar(out.into_series())
                                     },
                                     GroupsProxy::Slice { groups, .. } => {
-                                        // Slice and use computed null count
-                                        let out: IdxCa = groups
-                                            .iter()
-                                            .map(|g| {
-                                                let start = g[0];
-                                                let len = g[1];
-                                                len - s
-                                                    .slice(start as i64, len as usize)
-                                                    .null_count()
-                                                    as IdxSize
-                                            })
-                                            .collect_ca_trusted_with_dtype(&keep_name, IDX_DTYPE);
+                                        let out: IdxCa = if matches!(s.dtype(), &DataType::Null) {
+                                            IdxCa::full(s.name(), 0, groups.len())
+                                        } else {
+                                            // Slice and use computed null count
+                                            groups
+                                                .iter()
+                                                .map(|g| {
+                                                    let start = g[0];
+                                                    let len = g[1];
+                                                    len - s
+                                                        .slice(start as i64, len as usize)
+                                                        .null_count()
+                                                        as IdxSize
+                                                })
+                                                .collect_ca_trusted_with_dtype(
+                                                    &keep_name, IDX_DTYPE,
+                                                )
+                                        };
                                         AggregatedScalar(out.into_series())
                                     },
                                 }
diff --git a/py-polars/tests/unit/operations/test_aggregations.py b/py-polars/tests/unit/operations/test_aggregations.py
index 64ea9551dffe4..ca3153cdfc1b6 100644
--- a/py-polars/tests/unit/operations/test_aggregations.py
+++ b/py-polars/tests/unit/operations/test_aggregations.py
@@ -530,3 +530,11 @@ def test_horizontal_mean_in_groupby_15115() -> None:
             }
         ),
     )
+
+
+def test_group_count_over_null_column_15705() -> None:
+    df = pl.DataFrame(
+        {"a": [1, 1, 2, 2, 3, 3], "c": [None, None, None, None, None, None]}
+    )
+    out = df.group_by("a", maintain_order=True).agg(pl.col("c").count())
+    assert out["c"].to_list() == [0, 0, 0]