diff --git a/Doc/whatsnew/3.5.rst b/Doc/whatsnew/3.5.rst
index b18bcd2..2447898 100644
--- a/Doc/whatsnew/3.5.rst
+++ b/Doc/whatsnew/3.5.rst
@@ -192,3 +192,9 @@
   if it represented midnight in UTC.  This behavior was considered obscure and
   error-prone and has been removed in Python 3.5.  See :issue:`13936` for full
   details.
+
+* :meth:`unittest.TestCase.assertRaisesRegex` and
+  :meth:`~unittest.TestCase.assertWarnsRegex` now raise a :exc:`TypeError` if
+  the second argument is not a string or a compiled :mod:`regex`.  You may have
+  tests with an invalid second argument that have until 3.5 been falsely
+  passing which will now raise TypeErrors (:issue:`20145`).
diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
index 87fb02b..bedbc67 100644
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -143,7 +143,7 @@
                 self.obj_name = str(callable_obj)
         else:
             self.obj_name = None
-        if isinstance(expected_regex, (bytes, str)):
+        if expected_regex is not None:
             expected_regex = re.compile(expected_regex)
         self.expected_regex = expected_regex
         self.msg = None
diff --git a/Lib/unittest/test/test_case.py b/Lib/unittest/test/test_case.py
index 363390a..4932578 100644
--- a/Lib/unittest/test/test_case.py
+++ b/Lib/unittest/test/test_case.py
@@ -1126,6 +1126,18 @@
                 self.assertRaisesRegex, Exception, 'x',
                 lambda: None)
 
+    def testAssertRaisesRegexInvalidRegex(self):
+        # Issue 20145.
+        class MyExc(Exception):
+            pass
+        self.assertRaises(TypeError, self.assertRaisesRegex, MyExc, lambda: True)
+
+    def testAssertWarnsRegexInvalidRegex(self):
+        # Issue 20145.
+        class MyWarn(Warning):
+            pass
+        self.assertRaises(TypeError, self.assertWarnsRegex, MyWarn, lambda: True)
+
     def testAssertRaisesRegexMismatch(self):
         def Stub():
             raise Exception('Unexpected')
diff --git a/Misc/ACKS b/Misc/ACKS
index 8875389..b7404ca 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -546,6 +546,7 @@
 Albert Hofkamp
 Tomas Hoger
 Jonathan Hogg
+Kamilla Holanda
 Steve Holden
 Akintayo Holder
 Thomas Holenstein
diff --git a/Misc/NEWS b/Misc/NEWS
index 17e6ea5..0736f41 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -23,6 +23,9 @@
 Library
 -------
 
+- Issue #20145: `assertRaisesRegex` and `assertWarnsRegex` now raise a
+  TypeError if the second argument is not a string or compiled regex.
+
 - Issue #20633: Replace relative import by absolute import.
 
 - Issue #20980: Stop wrapping exception when using ThreadPool.