diff --git a/tests/hazmat/primitives/test_aead.py b/tests/hazmat/primitives/test_aead.py
index 2f0d52d82682..80850b689d35 100644
--- a/tests/hazmat/primitives/test_aead.py
+++ b/tests/hazmat/primitives/test_aead.py
@@ -37,8 +37,10 @@ def _aead_supported(cls):
         return False
 
 
-def large_mmap():
-    return mmap.mmap(-1, 2**32, prot=mmap.PROT_READ)
+def large_mmap(length: int = 2**32):
+    # Silencing mypy prot argument warning on Windows, even though this
+    # function is only used in non-Windows-based tests.
+    return mmap.mmap(-1, length, prot=mmap.PROT_READ)  # type: ignore[call-arg,attr-defined,unused-ignore]
 
 
 @pytest.mark.skipif(
diff --git a/tests/hazmat/primitives/test_ciphers.py b/tests/hazmat/primitives/test_ciphers.py
index 5fef25b86c0e..20dcb54d1b1d 100644
--- a/tests/hazmat/primitives/test_ciphers.py
+++ b/tests/hazmat/primitives/test_ciphers.py
@@ -4,7 +4,6 @@
 
 
 import binascii
-import mmap
 import os
 import sys
 
@@ -20,6 +19,7 @@
 )
 
 from ...utils import load_nist_vectors, load_vectors_from_file
+from .test_aead import large_mmap
 
 
 def test_deprecated_ciphers_import_with_warning():
@@ -255,7 +255,7 @@ def test_update_into_buffer_too_small_gcm(self, backend):
     sys.platform not in {"linux", "darwin"}, reason="mmap required"
 )
 def test_update_auto_chunking():
-    large_data = mmap.mmap(-1, 2**29 + 2**20, prot=mmap.PROT_READ)
+    large_data = large_mmap(length=2**29 + 2**20)
 
     key = b"\x00" * 16
     c = ciphers.Cipher(AES(key), modes.ECB())