diff --git a/tests/test_cpython_interface.py b/tests/test_cpython_interface.py
index 12ae4d4..3210479 100644
--- a/tests/test_cpython_interface.py
+++ b/tests/test_cpython_interface.py
@@ -142,10 +142,20 @@ def test_small_example_overrun(self):
         s = encoder.encode(0, 1024)
         minlen = len(s)
         for length in range(minlen):
-            with pytest.raises(ValueError, match="-101"):
+            with pytest.raises(_vcztools.VczBufferTooSmall, match="-101"):
                 encoder.encode(0, length)
         assert s == encoder.encode(0, minlen)
 
+    @pytest.mark.parametrize("variant", [0, 999, 333, 501])
+    def test_large_example_overrun(self, variant):
+        encoder = example_encoder(1000, 100)
+        s = encoder.encode(variant, 1024 * 1024)
+        minlen = len(s)
+        for length in range(minlen):
+            with pytest.raises(_vcztools.VczBufferTooSmall, match="-101"):
+                encoder.encode(variant, length)
+        assert s == encoder.encode(variant, minlen)
+
 
 class TestFixedFieldInputChecking:
     @pytest.mark.parametrize("name", FIXED_FIELD_NAMES)
diff --git a/vcztools/_vcztoolsmodule.c b/vcztools/_vcztoolsmodule.c
index 5cc8378..0e80f7f 100644
--- a/vcztools/_vcztoolsmodule.c
+++ b/vcztools/_vcztoolsmodule.c
@@ -17,11 +17,20 @@ typedef struct {
 } VcfEncoder;
 // clang-format on
 
+static PyObject *VczBufferTooSmall;
+
 static void
 handle_library_error(int err)
 {
-    // TODO generate string messages.
-    PyErr_Format(PyExc_ValueError, "Error occured: %d: ", err);
+    switch (err) {
+        case VCZ_ERR_BUFFER_OVERFLOW:
+            PyErr_Format(
+                VczBufferTooSmall, "Error: %d; specified buffer size is too small", err);
+            break;
+        // TODO handle the other error types.
+        default:
+            PyErr_Format(PyExc_ValueError, "Error occured: %d: ", err);
+    }
 }
 
 static FILE *
@@ -576,6 +585,10 @@ PyInit__vcztools(void)
     /* Initialise numpy */
     import_array();
 
+    VczBufferTooSmall = PyErr_NewException("_vcztools.VczBufferTooSmall", NULL, NULL);
+    Py_INCREF(VczBufferTooSmall);
+    PyModule_AddObject(module, "VczBufferTooSmall", VczBufferTooSmall);
+
     /* VcfEncoder type */
     if (PyType_Ready(&VcfEncoderType) < 0) {
         return NULL;
diff --git a/vcztools/vcf_writer.py b/vcztools/vcf_writer.py
index b383df1..54fe78e 100644
--- a/vcztools/vcf_writer.py
+++ b/vcztools/vcf_writer.py
@@ -258,9 +258,18 @@ def c_chunk_to_vcf(root, v_chunk, contigs, filters, output):
         if len(array.shape) == 2:
             array = array.reshape((num_variants, num_samples, 1))
         encoder.add_format_field(name, array)
-
+    # TODO: (1) make a guess at this based on number of fields and samples,
+    # and (2) log a DEBUG message when we have to double.
+    buflen = 1024
     for j in range(num_variants):
-        line = encoder.encode(j, 2**20)
+        failed = True
+        while failed:
+            try:
+                line = encoder.encode(j, buflen)
+                failed = False
+            except _vcztools.VczBufferTooSmall:
+                buflen *= 2
+                # print("Bumping buflen to", buflen)
         print(line, file=output)