diff --git a/test/README.md b/test/README.md
index 1c3303435db517..8d8cec44919720 100644
--- a/test/README.md
+++ b/test/README.md
@@ -290,8 +290,11 @@ Array of IPV6 hosts.
 ### mustCall(fn[, expected])
 * fn [<Function>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function)
 * expected [<Number>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type) default = 1
+* return [<Function>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function)
 
-Number of times `fn` should be called.
+Returns a function that calls `fn`. If the returned function has not been called
+exactly `expected` number of times when the test is complete, then the test will
+fail.
 
 ### nodeProcessAborted(exitCode, signal)
 * `exitCode` [<Number>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type)