diff --git a/src/cryptography/hazmat/primitives/kdf/hkdf.py b/src/cryptography/hazmat/primitives/kdf/hkdf.py
index 82ed9b1..964ac2c 100644
--- a/src/cryptography/hazmat/primitives/kdf/hkdf.py
+++ b/src/cryptography/hazmat/primitives/kdf/hkdf.py
@@ -30,7 +30,7 @@
             raise TypeError("salt must be bytes.")
 
         if salt is None:
-            salt = b"\x00" * (self._algorithm.digest_size // 8)
+            salt = b"\x00" * self._algorithm.digest_size
 
         self._salt = salt