diff --git a/src/blosc2/blosc2_ext.pyx b/src/blosc2/blosc2_ext.pyx
index de90e254c..3b99798e0 100644
--- a/src/blosc2/blosc2_ext.pyx
+++ b/src/blosc2/blosc2_ext.pyx
@@ -4184,16 +4184,17 @@ cdef class SChunk:
             len_data = buf.len
             nchunks = (len_data + chunksize - 1) // chunksize if len_data > 0 else 0
             len_chunk = chunksize
-            for i in range(nchunks):
-                if i == (nchunks - 1):
-                    len_chunk = len_data - i * chunksize
-                index = i * chunksize
-                rc = _store_buffer(self.schunk, _WRITE_APPEND, i, buf_ptr + index, len_chunk)
-                if rc < 0:
-                    PyBuffer_Release(&buf)
-                    _raise_callback_error()
-                    raise RuntimeError("Could not compress and append the chunk")
-            PyBuffer_Release(&buf)
+            try:
+                for i in range(nchunks):
+                    if i == (nchunks - 1):
+                        len_chunk = len_data - i * chunksize
+                    index = i * chunksize
+                    rc = _store_buffer(self.schunk, _WRITE_APPEND, i, buf_ptr + index, len_chunk)
+                    if rc < 0:
+                        _raise_callback_error()
+                        raise RuntimeError("Could not compress and append the chunk")
+            finally:
+                PyBuffer_Release(&buf)
 
     @property
     def c_schunk(self):
diff --git a/tests/test_schunk_data_writes.py b/tests/test_schunk_data_writes.py
index 887689daf..147bcc2ac 100644
--- a/tests/test_schunk_data_writes.py
+++ b/tests/test_schunk_data_writes.py
@@ -77,9 +77,9 @@ def test_data_writes(storage, releasegil):
             np.testing.assert_array_equal(got, exp)
 
 
-def test_data_writes_reach_prefilter(storage):
+def test_data_writes_reach_prefilter(storage, releasegil):
     # The prefilter computes each item from its position, so it must see the right chunk number.
-    # With the GIL held only: Python prefilters crash with set_releasegil(True).
+    # It runs Python, so the writes keep the GIL even with set_releasegil(True).
     schunk = new_schunk(storage, nthreads=1)
 
     @schunk.prefilter(np.int32, np.int32)