diff --git a/lib/operation.js b/lib/operation.js
index b57011fe..c86b382c 100644
--- a/lib/operation.js
+++ b/lib/operation.js
@@ -186,7 +186,7 @@ const blur = function blur (sigma) {
  * // to the top, left and right edges and 20 to the bottom edge
  * sharp(input)
  *   .resize(140)
- *   .background({r: 0, g: 0, b: 0, a: 0})
+ *   .background({r: 0, g: 0, b: 0, alpha: 0})
  *   .extend({top: 10, bottom: 20, left: 10, right: 10})
  *   ...
  *