diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index 39615051d4034fe42ee646012f4a85473ccb1923..a5fe16ae0a3f8f46b38a1afd4dfafc4bb1f28457 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -98,7 +98,7 @@ application. Accepts a valid week day symbol (e.g. `:monday`).
 
 * `config.exceptions_app` sets the exceptions application invoked by the ShowException middleware when an exception happens. Defaults to `ActionDispatch::PublicExceptions.new(Rails.public_path)`.
 
-* `config.debug_exception_response_format` sets the format used in responses when errors occur in development mode.
+* `config.debug_exception_response_format` sets the format used in responses when errors occur in development mode. Defaults to `:api` for API only apps and `:default` for normal apps.
 
 * `config.file_watcher` is the class used to detect file updates in the file system when `config.reload_classes_only_on_change` is true. Rails ships with `ActiveSupport::FileUpdateChecker`, the default, and `ActiveSupport::EventedFileUpdateChecker` (this one depends on the [listen](https://github.com/guard/listen) gem). Custom classes must conform to the `ActiveSupport::FileUpdateChecker` API.