diff --git a/docs/hazmat/primitives/twofactor.rst b/docs/hazmat/primitives/twofactor.rst
index 0e78143..3912d48 100644
--- a/docs/hazmat/primitives/twofactor.rst
+++ b/docs/hazmat/primitives/twofactor.rst
@@ -50,7 +50,8 @@
     :raises TypeError: This is raised if the provided ``algorithm`` is not
         :class:`~cryptography.hazmat.primitives.hashes.SHA1()`,
         :class:`~cryptography.hazmat.primitives.hashes.SHA256()` or
-        :class:`~cryptography.hazmat.primitives.hashes.SHA512()`.
+        :class:`~cryptography.hazmat.primitives.hashes.SHA512()` or if the
+        ``length`` parameter is not an integer.
 
     .. method:: generate(counter)
 
@@ -145,7 +146,8 @@
     :raises TypeError: This is raised if the provided ``algorithm`` is not
         :class:`~cryptography.hazmat.primitives.hashes.SHA1()`,
         :class:`~cryptography.hazmat.primitives.hashes.SHA256()` or
-        :class:`~cryptography.hazmat.primitives.hashes.SHA512()`.
+        :class:`~cryptography.hazmat.primitives.hashes.SHA512()` or if the
+        ``length`` parameter is not an integer.
 
     .. method:: generate(time)