1 files changed, 7 insertions, 0 deletions
diff --git a/Lib/test/test_gzip.py b/Lib/test/test_gzip.py
index c94d54e..034acb0 100644
--- a/Lib/test/test_gzip.py
+++ b/Lib/test/test_gzip.py
@@ -396,6 +396,13 @@ class TestGzip(BaseTest):
         with gzip.GzipFile(fileobj=io.BytesIO(gzdata)) as f:
             self.assertEqual(f.read(), b'Test')
 
+    def test_prepend_error(self):
+        # See issue #20875
+        with gzip.open(self.filename, "wb") as f:
+            f.write(data1)
+        with gzip.open(self.filename, "rb") as f:
+            f.fileobj.prepend()
+
 class TestOpen(BaseTest):
     def test_binary_modes(self):
         uncompressed = data1 * 50