diff --git a/countBy.js b/countBy.js
index fb5d44752..f72ece75b 100644
--- a/countBy.js
+++ b/countBy.js
@@ -22,6 +22,7 @@ const hasOwnProperty = Object.prototype.hasOwnProperty
  */
 function countBy(collection, iteratee) {
   return reduce(collection, (result, value, key) => {
+    key = iteratee(value)
     if (hasOwnProperty.call(result, key)) {
       ++result[key]
     } else {