diff --git a/Include/internal/pycore_blocks_output_buffer.h b/Include/internal/pycore_blocks_output_buffer.h
index 016e7a18665859..322c1e93344ba3 100644
--- a/Include/internal/pycore_blocks_output_buffer.h
+++ b/Include/internal/pycore_blocks_output_buffer.h
@@ -242,9 +242,12 @@ static inline PyObject *
 _BlocksOutputBuffer_Finish(_BlocksOutputBuffer *buffer,
                            const Py_ssize_t avail_out)
 {
+    PyObject *obj;
     assert(buffer->writer != NULL);
-    return PyBytesWriter_FinishWithSize(buffer->writer,
-                                        buffer->allocated - avail_out);
+    obj = PyBytesWriter_FinishWithSize(buffer->writer,
+                                       buffer->allocated - avail_out);
+    buffer->writer = NULL;
+    return obj;
 }
 
 /* Clean up the buffer when an error occurred. */
diff --git a/Misc/NEWS.d/next/Library/2026-04-17-16-31-58.gh-issue-148688.vVugFn.rst b/Misc/NEWS.d/next/Library/2026-04-17-16-31-58.gh-issue-148688.vVugFn.rst
new file mode 100644
index 00000000000000..1e367716e5a0a7
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-04-17-16-31-58.gh-issue-148688.vVugFn.rst
@@ -0,0 +1,2 @@
+:mod:`bz2`, :mod:`compression.zstd`, :mod:`lzma`, :mod:`zlib`: Fix a double
+free on memory allocation failure. Patch by Victor Stinner.