diff --git a/src/auto/injector.js b/src/auto/injector.js
index fb1dc3960a26..80db9ee68e52 100644
--- a/src/auto/injector.js
+++ b/src/auto/injector.js
@@ -179,7 +179,7 @@ function annotate(fn, strictDi, name) {
  * Return an instance of the service.
  *
  * @param {string} name The name of the instance to retrieve.
- * @param {string} caller An optional string to provide the origin of the function call for error messages.
+ * @param {string=} caller An optional string to provide the origin of the function call for error messages.
  * @return {*} The instance.
  */