diff --git a/tests/test_cpython_interface.py b/tests/test_cpython_interface.py
index a2c5bf6..533c64c 100644
--- a/tests/test_cpython_interface.py
+++ b/tests/test_cpython_interface.py
@@ -47,6 +47,17 @@ def example_format_data(num_variants, num_samples):
     return d
 
 
+def example_gt_data(num_variants, num_samples, ploidy=2):
+    return {
+        "gt": np.arange(num_variants * num_samples * ploidy)
+        .astype(np.int32)
+        .reshape(num_variants, num_samples, ploidy),
+        "gt_phased": (np.arange(num_variants * num_samples) % 2)
+        .reshape(num_variants, num_samples)
+        .astype(bool),
+    }
+
+
 def example_encoder(num_variants=1, num_samples=0, add_info=True):
     encoder = _vcztools.VcfEncoder(
         num_variants, num_samples, **example_fixed_data(num_variants, num_samples)
@@ -57,6 +68,9 @@ def example_encoder(num_variants=1, num_samples=0, add_info=True):
     if num_samples > 0:
         for name, data in example_format_data(num_variants, num_samples).items():
             encoder.add_format_field(name, data)
+
+        gt_data = example_gt_data(num_variants, num_samples)
+        encoder.add_gt_field(gt_data["gt"], gt_data["gt_phased"])
     # import sys
     # encoder.print_state(sys.stdout)
     return encoder
@@ -330,13 +344,22 @@ def test_add_gt_field_unsupported_width(self):
                 np.zeros((1, 1, 1), dtype=np.int64), np.zeros((1, 1), dtype=bool)
             )
 
+    def test_add_gt_field_zero_ploidy(self):
+        encoder = example_encoder(1, 1)
+        with pytest.raises(ValueError, match="-204"):
+            encoder.add_gt_field(
+                np.zeros((1, 1, 0), dtype=np.int64), np.zeros((1, 1), dtype=bool)
+            )
+
 
 class TestArrays:
     def test_stored_data_equal(self):
         num_variants = 20
         num_samples = 10
         fixed_data = example_fixed_data(num_variants, num_samples)
+        gt_data = example_gt_data(num_variants, num_samples)
         encoder = _vcztools.VcfEncoder(num_variants, num_samples, **fixed_data)
+        encoder.add_gt_field(gt_data["gt"], gt_data["gt_phased"])
         info_data = example_info_data(num_variants)
         format_data = example_format_data(num_variants, num_samples)
         for name, data in info_data.items():
@@ -346,7 +369,7 @@ def test_stored_data_equal(self):
         all_data = {**fixed_data}
         for name, array in info_data.items():
             all_data[f"INFO/{name}"] = array
-        for name, array in format_data.items():
+        for name, array in {**format_data, **gt_data}.items():
             all_data[f"FORMAT/{name}"] = array
         encoder_arrays = encoder.arrays
         assert set(encoder.arrays.keys()) == set(all_data.keys())
diff --git a/vcztools/_vcztoolsmodule.c b/vcztools/_vcztoolsmodule.c
index 3130628..fa09679 100644
--- a/vcztools/_vcztoolsmodule.c
+++ b/vcztools/_vcztoolsmodule.c
@@ -174,7 +174,8 @@ np_type_to_vcz_type(const char *name, PyArrayObject *array)
 }
 
 static int
-check_dtype(const char *name, PyArrayObject *array, int type) {
+check_dtype(const char *name, PyArrayObject *array, int type)
+{
     if (PyArray_DTYPE(array)->type_num != type) {
         PyErr_Format(PyExc_ValueError, "Wrong dtype for %s", name);
         return -1;
@@ -411,8 +412,7 @@ VcfEncoder_add_gt_field(VcfEncoder *self, PyObject *args)
         goto out;
     }
     if (PyArray_DTYPE(gt)->kind != 'i') {
-        PyErr_Format(
-            PyExc_ValueError, "Array 'gt' has unsupported array dtype");
+        PyErr_Format(PyExc_ValueError, "Array 'gt' has unsupported array dtype");
         goto out;
     }
     if (check_dtype("gt_phased", gt_phased, NPY_BOOL) != 0) {