diff --git a/cryptography/hazmat/primitives/kdf/hkdf.py b/cryptography/hazmat/primitives/kdf/hkdf.py
index 44e1481..d49cc5b 100644
--- a/cryptography/hazmat/primitives/kdf/hkdf.py
+++ b/cryptography/hazmat/primitives/kdf/hkdf.py
@@ -34,6 +34,57 @@
 
         self._algorithm = algorithm
 
+        if isinstance(salt, six.text_type):
+            raise TypeError(
+                "Unicode-objects must be encoded before using them as a salt.")
+
+        if salt is None:
+            salt = b"\x00" * (self._algorithm.digest_size // 8)
+
+        self._salt = salt
+
+        self._backend = backend
+
+        self._used = False
+
+        self._hkdf_expand = HKDFExpand(self._algorithm, length, info, backend)
+
+    def _extract(self, key_material):
+        h = hmac.HMAC(self._salt, self._algorithm, backend=self._backend)
+        h.update(key_material)
+        return h.finalize()
+
+    def derive(self, key_material):
+        if isinstance(key_material, six.text_type):
+            raise TypeError(
+                "Unicode-objects must be encoded before using them as key "
+                "material."
+            )
+
+        if self._used:
+            raise AlreadyFinalized
+
+        self._used = True
+        return self._hkdf_expand.derive(self._extract(key_material))
+
+    def verify(self, key_material, expected_key):
+        if not constant_time.bytes_eq(self.derive(key_material), expected_key):
+            raise InvalidKey
+
+
+@utils.register_interface(interfaces.KeyDerivationFunction)
+class HKDFExpand(HKDF):
+    def __init__(self, algorithm, length, info, backend):
+        if not isinstance(backend, HMACBackend):
+            raise UnsupportedAlgorithm(
+                "Backend object does not implement HMACBackend",
+                _Reasons.BACKEND_MISSING_INTERFACE
+            )
+
+        self._algorithm = algorithm
+
+        self._backend = backend
+
         max_length = 255 * (algorithm.digest_size // 8)
 
         if length > max_length:
@@ -44,15 +95,6 @@
 
         self._length = length
 
-        if isinstance(salt, six.text_type):
-            raise TypeError(
-                "Unicode-objects must be encoded before using them as a salt.")
-
-        if salt is None:
-            salt = b"\x00" * (self._algorithm.digest_size // 8)
-
-        self._salt = salt
-
         if isinstance(info, six.text_type):
             raise TypeError(
                 "Unicode-objects must be encoded before using them as info.")
@@ -61,15 +103,9 @@
             info = b""
 
         self._info = info
-        self._backend = backend
 
         self._used = False
 
-    def _extract(self, key_material):
-        h = hmac.HMAC(self._salt, self._algorithm, backend=self._backend)
-        h.update(key_material)
-        return h.finalize()
-
     def _expand(self, key_material):
         output = [b""]
         counter = 1
@@ -87,29 +123,6 @@
     def derive(self, key_material):
         if isinstance(key_material, six.text_type):
             raise TypeError(
-                "Unicode-objects must be encoded before using them as key "
-                "material."
-            )
-
-        if self._used:
-            raise AlreadyFinalized
-
-        self._used = True
-        return self._expand(self._extract(key_material))
-
-    def verify(self, key_material, expected_key):
-        if not constant_time.bytes_eq(self.derive(key_material), expected_key):
-            raise InvalidKey
-
-
-@utils.register_interface(interfaces.KeyDerivationFunction)
-class HKDFExpand(HKDF):
-    def __init__(self, algorithm, length, info, backend):
-        HKDF.__init__(self, algorithm, length, None, info, backend)
-
-    def derive(self, key_material):
-        if isinstance(key_material, six.text_type):
-            raise TypeError(
                 "Unicode-objects must be encoded before using them as key"
                 "material."
             )
diff --git a/tests/hazmat/primitives/test_hkdf.py b/tests/hazmat/primitives/test_hkdf.py
index bee4217..598f09f 100644
--- a/tests/hazmat/primitives/test_hkdf.py
+++ b/tests/hazmat/primitives/test_hkdf.py
@@ -214,3 +214,6 @@
 
     with raises_unsupported_algorithm(_Reasons.BACKEND_MISSING_INTERFACE):
         HKDF(hashes.SHA256(), 16, None, None, pretend_backend)
+
+    with raises_unsupported_algorithm(_Reasons.BACKEND_MISSING_INTERFACE):
+        HKDFExpand(hashes.SHA256(), 16, None, pretend_backend)
diff --git a/tests/hazmat/primitives/utils.py b/tests/hazmat/primitives/utils.py
index 6c3f4c9..7cf5efd 100644
--- a/tests/hazmat/primitives/utils.py
+++ b/tests/hazmat/primitives/utils.py
@@ -26,7 +26,7 @@
 from cryptography.hazmat.primitives import hashes, hmac
 from cryptography.hazmat.primitives.asymmetric import rsa
 from cryptography.hazmat.primitives.ciphers import Cipher
-from cryptography.hazmat.primitives.kdf.hkdf import HKDF
+from cryptography.hazmat.primitives.kdf.hkdf import HKDF, HKDFExpand
 from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC
 
 from ...utils import load_vectors_from_file
@@ -347,10 +347,9 @@
 
 
 def hkdf_expand_test(backend, algorithm, params):
-    hkdf = HKDF(
+    hkdf = HKDFExpand(
         algorithm,
         int(params["l"]),
-        salt=binascii.unhexlify(params["salt"]) or None,
         info=binascii.unhexlify(params["info"]) or None,
         backend=backend
     )