diff --git a/bio2zarr/vcf2zarr/vcz.py b/bio2zarr/vcf2zarr/vcz.py
index 742e9f61..8dfa9776 100644
--- a/bio2zarr/vcf2zarr/vcz.py
+++ b/bio2zarr/vcf2zarr/vcz.py
@@ -1139,7 +1139,7 @@ def create_index(self):
                 )
                 c_start_idx = c_end_idx + 1
 
-        index = np.array(index, dtype=np.int32)
+        index = np.array(index, dtype=pos.dtype)
         kwargs = {}
         if not zarr_v3():
             kwargs["dimension_separator"] = self.metadata.dimension_separator
@@ -1147,8 +1147,10 @@ def create_index(self):
             "region_index",
             data=index,
             shape=index.shape,
+            chunks=index.shape,
             dtype=index.dtype,
             compressor=numcodecs.Blosc("zstd", clevel=9, shuffle=0),
+            fill_value=None,
             **kwargs,
         )
         array.attrs["_ARRAY_DIMENSIONS"] = [
diff --git a/tests/test_vcf_examples.py b/tests/test_vcf_examples.py
index 3692fe44..06a0aaf5 100644
--- a/tests/test_vcf_examples.py
+++ b/tests/test_vcf_examples.py
@@ -438,6 +438,17 @@ def test_vcf_dimensions(self, ds):
     def test_vcf_field_description(self, ds, field, description):
         assert ds[field].attrs["description"] == description
 
+    def test_region_index(self, ds):
+        assert ds["region_index"].chunks == ((3,), (6,))
+        region_index = np.array(
+            [
+                [0, 0, 111, 112, 112, 2],
+                [0, 1, 14370, 1235237, 1235237, 6],
+                [0, 2, 10, 10, 11, 1],
+            ]
+        )
+        nt.assert_array_equal(ds["region_index"], region_index)
+
 
 class TestSmallExampleLocalAlleles:
     data_path = "tests/data/vcf/sample.vcf.gz"
diff --git a/tests/test_vcz.py b/tests/test_vcz.py
index 19c022bd..11e570ad 100644
--- a/tests/test_vcz.py
+++ b/tests/test_vcz.py
@@ -250,6 +250,20 @@ def test_genotype_dtype(self, tmp_path, icf_path, dtype):
         root = zarr.open(zarr_path)
         assert root["call_genotype"].dtype == dtype
 
+    @pytest.mark.parametrize("dtype", ["i4", "i8"])
+    def test_region_index_dtype(self, tmp_path, icf_path, dtype):
+        zarr_path = tmp_path / "zarr"
+        icf = vcf2zarr.IntermediateColumnarFormat(icf_path)
+        schema = vcf2zarr.VcfZarrSchema.generate(icf)
+        schema.field_map()["variant_position"].dtype = dtype
+        schema_path = tmp_path / "schema"
+        with open(schema_path, "w") as f:
+            f.write(schema.asjson())
+        vcf2zarr.encode(icf_path, zarr_path, schema_path=schema_path)
+        root = zarr.open(zarr_path)
+        assert root["variant_position"].dtype == dtype
+        assert root["region_index"].dtype == dtype
+
 
 def get_field_dict(a_schema, name):
     d = a_schema.asdict()