diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
index b0cb44a..644fe5b 100644
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -122,8 +122,6 @@
             return True
 
         expected_regexp = self.expected_regexp
-        if isinstance(expected_regexp, basestring):
-            expected_regexp = re.compile(expected_regexp)
         if not expected_regexp.search(str(exc_value)):
             raise self.failureException('"%s" does not match "%s"' %
                      (expected_regexp.pattern, str(exc_value)))
@@ -986,6 +984,8 @@
             args: Extra args.
             kwargs: Extra kwargs.
         """
+        if expected_regexp is not None:
+            expected_regexp = re.compile(expected_regexp)
         context = _AssertRaisesContext(expected_exception, self, expected_regexp)
         if callable_obj is None:
             return context
diff --git a/Lib/unittest/test/test_case.py b/Lib/unittest/test/test_case.py
index 42a753b..b69fdb3 100644
--- a/Lib/unittest/test/test_case.py
+++ b/Lib/unittest/test/test_case.py
@@ -979,6 +979,12 @@
                 self.assertRaisesRegexp, Exception, u'x',
                 lambda: None)
 
+    def testAssertRaisesRegexpInvalidRegexp(self):
+        # Issue 20145.
+        class MyExc(Exception):
+            pass
+        self.assertRaises(TypeError, self.assertRaisesRegexp, MyExc, lambda: True)
+
     def testAssertRaisesRegexpMismatch(self):
         def Stub():
             raise Exception('Unexpected')