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 68617ce..0eb8267 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 2a7cd3b..b00a017 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -24,6 +24,9 @@
 Library
 -------
 
+- Issue #20145: `assertRaisesRegex` and `assertWarnsRegex` now raise a
+  TypeError if the second argument is not a string or compiled regex.
+
 - Issue #21058: Fix a leak of file descriptor in
   :func:`tempfile.NamedTemporaryFile`, close the file descriptor if
   :func:`io.open` fails