diff --git a/tests/test_igzip_threaded.py b/tests/test_igzip_threaded.py
index dd26e95e..806783ad 100644
--- a/tests/test_igzip_threaded.py
+++ b/tests/test_igzip_threaded.py
@@ -229,14 +229,16 @@ def test_close_writer(threads):
     f.close()
 
 
+# io.BufferedReader.writable() and io.BufferedWriter.readable() are always
+# False, so check the raw streams.
 def test_reader_not_writable():
     with igzip_threaded.open(TEST_FILE, "rb") as f:
-        assert not f.writable()
+        assert not f.raw.writable()
 
 
 def test_writer_not_readable():
     with igzip_threaded.open(io.BytesIO(), "wb") as f:
-        assert not f.readable()
+        assert not f.raw.readable()
 
 
 def test_writer_wrong_level():