diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index bf1ffe8992049ea816dcb5904b3f64d56ecd2ed1..5729a98b994fb19194f9aad98750ca0946bc8c93 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Fix behavior of JSON encoding for `Exception`.
+
+    *namusyaka*
+
 *   Make `number_to_phone` format number with regexp pattern.
 
         number_to_phone(18812345678, pattern: /(\d{3})(\d{4})(\d{4})/)
diff --git a/activesupport/lib/active_support/core_ext/object/json.rb b/activesupport/lib/active_support/core_ext/object/json.rb
index 0db787010c81a80b2e6eeafb78b270ca0001fca4..d49b2fbe546e5a490c38c6e47b1f9cbbc18033bd 100644
--- a/activesupport/lib/active_support/core_ext/object/json.rb
+++ b/activesupport/lib/active_support/core_ext/object/json.rb
@@ -197,3 +197,9 @@ def as_json(options = nil)
     { :exitstatus => exitstatus, :pid => pid }
   end
 end
+
+class Exception
+  def as_json(options = nil)
+    to_s
+  end
+end
diff --git a/activesupport/test/json/encoding_test.rb b/activesupport/test/json/encoding_test.rb
index 9f4b62fd8bea3c4cb4737ab51621d6a2b93139b9..5fc2e1633630a61e4c1d8bc859753e5d6253c5c2 100644
--- a/activesupport/test/json/encoding_test.rb
+++ b/activesupport/test/json/encoding_test.rb
@@ -422,6 +422,11 @@ def test_twz_to_json_when_wrapping_a_date_time
     assert_equal '"1999-12-31T19:00:00.000-05:00"', ActiveSupport::JSON.encode(time)
   end
 
+  def test_exception_to_json
+    exception = Exception.new("foo")
+    assert_equal '"foo"', ActiveSupport::JSON.encode(exception)
+  end
+
   protected
 
     def object_keys(json_object)