Added backend check to kdf primitives
diff --git a/docs/hazmat/primitives/key-derivation-functions.rst b/docs/hazmat/primitives/key-derivation-functions.rst
index 851dbb0..174b68d 100644
--- a/docs/hazmat/primitives/key-derivation-functions.rst
+++ b/docs/hazmat/primitives/key-derivation-functions.rst
@@ -84,6 +84,10 @@
:class:`~cryptography.hazmat.backends.interfaces.PBKDF2HMACBackend`
provider.
+ :raises cryptography.exceptions.UnsupportedInterface: This is raised if the
+ provided ``backend`` does not implement
+ :class:`~cryptography.hazmat.backends.interfaces.PBKDF2HMACBackend`
+
.. method:: derive(key_material)
:param bytes key_material: The input key material. For PBKDF2 this
@@ -183,6 +187,10 @@
:class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
provider.
+ :raises cryptography.exceptions.UnsupportedInterface: This is raised if the
+ provided ``backend`` does not implement
+ :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
+
.. method:: derive(key_material)
:param bytes key_material: The input key material.