diff --git a/cryptography/primitives/block/ciphers.py b/cryptography/primitives/block/ciphers.py
index 7363aeb..f10a349 100644
--- a/cryptography/primitives/block/ciphers.py
+++ b/cryptography/primitives/block/ciphers.py
@@ -64,3 +64,13 @@
     def __init__(self, key):
         super(TripleDES, self).__init__()
         self.key = key
+
+        # Verify that the key size matches the expected key size
+        if self.key_size not in self.key_sizes:
+            raise ValueError("Invalid key size ({0}) for {1}".format(
+                self.key_size, self.name
+            ))
+
+    @property
+    def key_size(self):
+        return len(self.key) * 8
diff --git a/tests/primitives/test_nist.py b/tests/primitives/test_nist.py
index 1c23fca..8c8a381 100644
--- a/tests/primitives/test_nist.py
+++ b/tests/primitives/test_nist.py
@@ -26,6 +26,16 @@
 from ..utils import load_nist_vectors_from_file
 
 
+def load_3des_nist_vectors_from_file(path, op):
+    vectors = load_nist_vectors_from_file(path, op)
+    for vector in vectors:
+        vector["ciphertext"] = vector["ciphertext3"]
+        del vector["ciphertext1"]
+        del vector["ciphertext2"]
+        del vector["ciphertext3"]
+    return vectors
+
+
 class TestAES_CBC(object):
     test_KAT = generate_encrypt_test(
         lambda path: load_nist_vectors_from_file(path, "ENCRYPT"),
@@ -182,7 +192,7 @@
     )
 
     test_KAT2 = generate_encrypt_test(
-        lambda path: load_nist_vectors_from_file(path, "ENCRYPT"),
+        lambda path: load_3des_nist_vectors_from_file(path, "ENCRYPT"),
         os.path.join("3DES", "KAT"),
         [
             "TCBCIpermop.rsp",