diff --git a/cryptography/hazmat/bindings/openssl/rsa.py b/cryptography/hazmat/bindings/openssl/rsa.py
index 359305c..f895cd0 100644
--- a/cryptography/hazmat/bindings/openssl/rsa.py
+++ b/cryptography/hazmat/bindings/openssl/rsa.py
@@ -37,6 +37,7 @@
 static const int RSA_F4;
 
 static const int Cryptography_HAS_PSS_PADDING;
+static const int Cryptography_HAS_MGF1_MD;
 """
 
 FUNCTIONS = """
@@ -70,6 +71,7 @@
 MACROS = """
 int EVP_PKEY_CTX_set_rsa_padding(EVP_PKEY_CTX *, int);
 int EVP_PKEY_CTX_set_rsa_pss_saltlen(EVP_PKEY_CTX *, int);
+int EVP_PKEY_CTX_set_rsa_mgf1_md(EVP_PKEY_CTX *, EVP_MD *);
 """
 
 CUSTOMIZATIONS = """
@@ -82,6 +84,12 @@
 int (*EVP_PKEY_CTX_set_rsa_pss_saltlen)(EVP_PKEY_CTX *, int) = NULL;
 static const long RSA_PKCS1_PSS_PADDING = 0;
 #endif
+#if OPENSSL_VERSION_NUMBER >= 0x1000100f
+static const long Cryptography_HAS_MGF1_MD = 1;
+#else
+static const long Cryptography_HAS_MGF1_MD = 0;
+int (*EVP_PKEY_CTX_set_rsa_mgf1_md)(EVP_PKEY_CTX *, EVP_MD *) = NULL;
+#endif
 """
 
 CONDITIONAL_NAMES = {
@@ -92,4 +100,7 @@
     "Cryptography_HAS_PSS_PADDING": [
         "RSA_PKCS1_PSS_PADDING",
     ],
+    "Cryptography_HAS_MGF1_MD": [
+        "EVP_PKEY_CTX_set_rsa_mgf1_md",
+    ],
 }
diff --git a/cryptography/hazmat/primitives/twofactor/hotp.py b/cryptography/hazmat/primitives/twofactor/hotp.py
index 88bde71..8326022 100644
--- a/cryptography/hazmat/primitives/twofactor/hotp.py
+++ b/cryptography/hazmat/primitives/twofactor/hotp.py
@@ -27,6 +27,9 @@
         if len(key) < 16:
             raise ValueError("Key length has to be at least 128 bits.")
 
+        if not isinstance(length, six.integer_types):
+            raise TypeError("Length parameter must be an integer type")
+
         if length < 6 or length > 8:
             raise ValueError("Length of HOTP has to be between 6 to 8.")
 
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)
 
diff --git a/tests/hazmat/primitives/twofactor/test_hotp.py b/tests/hazmat/primitives/twofactor/test_hotp.py
index 4c726b7..0f8c4a5 100644
--- a/tests/hazmat/primitives/twofactor/test_hotp.py
+++ b/tests/hazmat/primitives/twofactor/test_hotp.py
@@ -87,3 +87,9 @@
 
         with pytest.raises(InvalidToken):
             hotp.verify(b"123456", counter)
+
+    def test_length_not_int(self, backend):
+        secret = b"12345678901234567890"
+
+        with pytest.raises(TypeError):
+            HOTP(secret, b"foo", SHA1(), backend)