diff --git a/bio2zarr/cli.py b/bio2zarr/cli.py
index 54f344d8..670a323f 100644
--- a/bio2zarr/cli.py
+++ b/bio2zarr/cli.py
@@ -58,6 +58,13 @@ def list_commands(self, ctx):
     help="Force overwriting of existing directories",
 )
 
+one_based = click.option(
+    "--one-based",
+    is_flag=True,
+    flag_value=True,
+    help="Partition indexes are interpreted as one-based",
+)
+
 version = click.version_option(version=f"{provenance.__version__}")
 
 worker_processes = click.option(
@@ -226,13 +233,18 @@ def dexplode_init(
 @icf_path
 @partition
 @verbose
-def dexplode_partition(icf_path, partition, verbose):
+@one_based
+def dexplode_partition(icf_path, partition, verbose, one_based):
     """
-    Convert a VCF partition to intermediate columnar format. Must be called *after*
-    the ICF path has been initialised with dexplode_init. Partition indexes must be
-    from 0 (inclusive) to the number of paritions returned by dexplode_init (exclusive).
+    Convert a VCF partition to intermediate columnar format. Must be called
+    after the ICF path has been initialised with dexplode_init. By default,
+    partition indexes are from 0 to the number of partitions N (returned by
+    dexplode_init), exclusive. If the --one-based option is specifed,
+    partition indexes are in the range 1 to N, inclusive.
     """
     setup_logging(verbose)
+    if one_based:
+        partition -= 1
     vcf.explode_partition(icf_path, partition)
 
 
@@ -371,14 +383,17 @@ def dencode_init(
 @zarr_path
 @partition
 @verbose
-def dencode_partition(zarr_path, partition, verbose):
-    """
-    Convert a partition from intermediate columnar format to VCF Zarr.
-    Must be called *after* the Zarr path has been initialised with dencode_init.
-    Partition indexes must be from 0 (inclusive) to the number of paritions
-    returned by dencode_init (exclusive).
+@one_based
+def dencode_partition(zarr_path, partition, verbose, one_based):
     """
+    Convert a partition from intermediate columnar format to VCF Zarr. Must be
+    called after the Zarr path has been initialised with dencode_init. By
+    default, partition indexes are from 0 to the number of partitions N
+    (returned by dencode_init), exclusive. If the --one-based option is
+    specifed, partition indexes are in the range 1 to N, inclusive."""
     setup_logging(verbose)
+    if one_based:
+        partition -= 1
     vcf.encode_partition(zarr_path, partition)
 
 
diff --git a/bio2zarr/vcf.py b/bio2zarr/vcf.py
index a2a090fd..c642ad3e 100644
--- a/bio2zarr/vcf.py
+++ b/bio2zarr/vcf.py
@@ -382,8 +382,8 @@ def sanitise_value_bool(buff, j, value):
 def sanitise_value_float_scalar(buff, j, value):
     x = value
     if value is None:
-        x = FLOAT32_MISSING
-    buff[j] = x
+        x = [FLOAT32_MISSING]
+    buff[j] = x[0]
 
 
 def sanitise_value_int_scalar(buff, j, value):
@@ -392,7 +392,7 @@ def sanitise_value_int_scalar(buff, j, value):
         # print("MISSING", INT_MISSING, INT_FILL)
         x = [INT_MISSING]
     else:
-        x = sanitise_int_array([value], ndmin=1, dtype=np.int32)
+        x = sanitise_int_array(value, ndmin=1, dtype=np.int32)
     buff[j] = x[0]
 
 
@@ -1148,9 +1148,7 @@ def explode(self, *, worker_processes=1, show_progress=False):
     def explode_partition(self, partition):
         self.load_metadata()
         if partition < 0 or partition >= self.num_partitions:
-            raise ValueError(
-                "Partition index must be in the range 0 <= index < num_partitions"
-            )
+            raise ValueError("Partition index not in the valid range")
         self.process_partition(partition)
 
     def finalise(self):
@@ -1801,9 +1799,7 @@ def partition_array_path(self, partition_index, name):
     def encode_partition(self, partition_index):
         self.load_metadata()
         if partition_index < 0 or partition_index >= self.num_partitions:
-            raise ValueError(
-                "Partition index must be in the range 0 <= index < num_partitions"
-            )
+            raise ValueError("Partition index not in the valid range")
         partition_path = self.wip_partition_path(partition_index)
         partition_path.mkdir(exist_ok=True)
         logger.info(f"Encoding partition {partition_index} to {partition_path}")
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 9a9421aa..f37a3fc7 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -314,6 +314,23 @@ def test_vcf_dexplode_partition(self, mocked, tmp_path):
             str(icf_path), 1, **DEFAULT_DEXPLODE_PARTITION_ARGS
         )
 
+    @mock.patch("bio2zarr.vcf.explode_partition")
+    def test_vcf_dexplode_partition_one_based(self, mocked, tmp_path):
+        runner = ct.CliRunner(mix_stderr=False)
+        icf_path = tmp_path / "icf"
+        icf_path.mkdir()
+        result = runner.invoke(
+            cli.vcf2zarr,
+            f"dexplode-partition {icf_path} 1 --one-based",
+            catch_exceptions=False,
+        )
+        assert result.exit_code == 0
+        assert len(result.stdout) == 0
+        assert len(result.stderr) == 0
+        mocked.assert_called_once_with(
+            str(icf_path), 0, **DEFAULT_DEXPLODE_PARTITION_ARGS
+        )
+
     @mock.patch("bio2zarr.vcf.explode_partition")
     def test_vcf_dexplode_partition_missing_dir(self, mocked, tmp_path):
         runner = ct.CliRunner(mix_stderr=False)
@@ -436,6 +453,23 @@ def test_vcf_dencode_partition(self, mocked, tmp_path):
             str(zarr_path), 1, **DEFAULT_DENCODE_PARTITION_ARGS
         )
 
+    @mock.patch("bio2zarr.vcf.encode_partition")
+    def test_vcf_dencode_partition_one_based(self, mocked, tmp_path):
+        runner = ct.CliRunner(mix_stderr=False)
+        zarr_path = tmp_path / "zarr"
+        zarr_path.mkdir()
+        result = runner.invoke(
+            cli.vcf2zarr,
+            f"dencode-partition {zarr_path} 1 --one-based",
+            catch_exceptions=False,
+        )
+        assert result.exit_code == 0
+        assert len(result.stdout) == 0
+        assert len(result.stderr) == 0
+        mocked.assert_called_once_with(
+            str(zarr_path), 0, **DEFAULT_DENCODE_PARTITION_ARGS
+        )
+
     @mock.patch("bio2zarr.vcf.encode_finalise")
     def test_vcf_dencode_finalise(self, mocked, tmp_path):
         runner = ct.CliRunner(mix_stderr=False)
@@ -489,7 +523,8 @@ def test_convert_plink(self, mocked):
 class TestVcfEndToEnd:
     vcf_path = "tests/data/vcf/sample.vcf.gz"
 
-    def test_dexplode(self, tmp_path):
+    @pytest.mark.parametrize("one_based", [False, True])
+    def test_dexplode(self, tmp_path, one_based):
         icf_path = tmp_path / "icf"
         runner = ct.CliRunner(mix_stderr=False)
         result = runner.invoke(
@@ -501,11 +536,11 @@ def test_dexplode(self, tmp_path):
         assert result.stdout.strip() == "3"
 
         for j in range(3):
-            result = runner.invoke(
-                cli.vcf2zarr,
-                f"dexplode-partition {icf_path} {j}",
-                catch_exceptions=False,
-            )
+            if one_based:
+                cmd = f"dexplode-partition {icf_path} {j + 1} --one-based"
+            else:
+                cmd = f"dexplode-partition {icf_path} {j}"
+            result = runner.invoke(cli.vcf2zarr, cmd, catch_exceptions=False)
             assert result.exit_code == 0
         result = runner.invoke(
             cli.vcf2zarr, f"dexplode-finalise {icf_path}", catch_exceptions=False
@@ -552,7 +587,8 @@ def test_encode(self, tmp_path):
         # Arbitrary check
         assert "variant_position" in result.stdout
 
-    def test_dencode(self, tmp_path):
+    @pytest.mark.parametrize("one_based", [False, True])
+    def test_dencode(self, tmp_path, one_based):
         icf_path = tmp_path / "icf"
         zarr_path = tmp_path / "zarr"
         runner = ct.CliRunner(mix_stderr=False)
@@ -569,12 +605,12 @@ def test_dencode(self, tmp_path):
         assert result.stdout.split()[0] == "3"
 
         for j in range(3):
-            result = runner.invoke(
-                cli.vcf2zarr,
-                f"dencode-partition {zarr_path} {j}",
-                catch_exceptions=False,
-            )
-        assert result.exit_code == 0
+            if one_based:
+                cmd = f"dencode-partition {zarr_path} {j + 1} --one-based"
+            else:
+                cmd = f"dencode-partition {zarr_path} {j}"
+            result = runner.invoke(cli.vcf2zarr, cmd, catch_exceptions=False)
+            assert result.exit_code == 0
 
         result = runner.invoke(
             cli.vcf2zarr, f"dencode-finalise {zarr_path}", catch_exceptions=False
diff --git a/tests/test_icf.py b/tests/test_icf.py
index 45800001..2cc21432 100644
--- a/tests/test_icf.py
+++ b/tests/test_icf.py
@@ -170,7 +170,7 @@ def test_double_explode_partition(self, tmp_path):
     def test_explode_partition_out_of_range(self, tmp_path, partition):
         icf_path = tmp_path / "x.icf"
         vcf.explode_init(icf_path, [self.data_path])
-        with pytest.raises(ValueError, match="Partition index must be in the range"):
+        with pytest.raises(ValueError, match="Partition index not in the valid range"):
             vcf.explode_partition(icf_path, partition)
 
     def test_explode_same_file_twice(self, tmp_path):
diff --git a/tests/test_vcf.py b/tests/test_vcf.py
index 40723dc1..c71b666d 100644
--- a/tests/test_vcf.py
+++ b/tests/test_vcf.py
@@ -502,5 +502,5 @@ def test_double_encode_partition(self, icf_path, tmp_path, caplog):
     def test_encode_partition_out_of_range(self, icf_path, tmp_path, partition):
         zarr_path = tmp_path / "x.zarr"
         vcf.encode_init(icf_path, zarr_path, 3, variants_chunk_size=3)
-        with pytest.raises(ValueError, match="Partition index must be in the range"):
+        with pytest.raises(ValueError, match="Partition index not in the valid range"):
             vcf.encode_partition(zarr_path, partition)