diff --git a/tlslite/utils/constanttime.py b/tlslite/utils/constanttime.py
index 60322c14..d4f5b1ce 100644
--- a/tlslite/utils/constanttime.py
+++ b/tlslite/utils/constanttime.py
@@ -170,7 +170,7 @@ def ct_check_cbc_mac_and_pad(data, mac, seqnumBytes, contentType, version):
     data_mac.update(compatHMAC(data[:start_pos]))
 
     # don't check past the array end (already checked to be >= zero)
-    end_pos = data_len - 1 - mac.digest_size
+    end_pos = data_len - mac.digest_size
 
     # calculate all possible
     for i in range(start_pos, end_pos): # constant for given overall length
diff --git a/unit_tests/test_tlslite_utils_constanttime.py b/unit_tests/test_tlslite_utils_constanttime.py
index 0edaf3f4..0a6446d0 100644
--- a/unit_tests/test_tlslite_utils_constanttime.py
+++ b/unit_tests/test_tlslite_utils_constanttime.py
@@ -16,6 +16,7 @@
 from hypothesis import given, example
 import hypothesis.strategies as st
 from tlslite.utils.compat import compatHMAC
+from tlslite.utils.cryptomath import getRandomBytes
 from tlslite.recordlayer import RecordLayer
 import tlslite.utils.tlshashlib as hashlib
 import hmac
@@ -266,6 +267,26 @@ def test_with_invalid_hash(self):
         self.assertFalse(ct_check_cbc_mac_and_pad(data, h, seqnum_bytes,
                                                   content_type, version))
 
+    @given(i=st.integers(1, 20))
+    def test_with_invalid_random_hash(self, i):
+        key = compatHMAC(getRandomBytes(20))
+        seqnum_bytes = bytearray(16)
+        content_type = 0x15
+        version = (3, 3)
+        application_data = getRandomBytes(63)
+        mac = hashlib.sha1
+
+        data = self.data_prepare(application_data, seqnum_bytes, content_type,
+                                 version, mac, key)
+        data[-i] ^= 0xff
+        padding = bytearray(b'\x00')
+        data += padding
+
+        h = hmac.new(key, digestmod=mac)
+        h.block_size = mac().block_size
+        self.assertFalse(ct_check_cbc_mac_and_pad(data, h, seqnum_bytes,
+                                                  content_type, version))
+
     def test_with_invalid_pad(self):
         key = compatHMAC(bytearray(20))
         seqnum_bytes = bytearray(16)