diff --git a/docs/api-operation.md b/docs/api-operation.md
index 85eb6197..5422a194 100644
--- a/docs/api-operation.md
+++ b/docs/api-operation.md
@@ -118,7 +118,8 @@ Merge alpha transparency channel, if any, with a background.
 
 ### Parameters
 
--   `options`  
+-   `options` **[Object][2]?** 
+    -   `options.background` **([String][3] \| [Object][2])** background colour, parsed by the [color][4] module, defaults to black. (optional, default `{r:0,g:0,b:0}`)
 
 Returns **Sharp** 
 
diff --git a/lib/operation.js b/lib/operation.js
index 5cb816ca..4eb86f5f 100644
--- a/lib/operation.js
+++ b/lib/operation.js
@@ -172,6 +172,7 @@ function blur (sigma) {
 
 /**
  * Merge alpha transparency channel, if any, with a background.
+ * @param {Object} [options]
  * @param {String|Object} [options.background={r: 0, g: 0, b: 0}] - background colour, parsed by the [color](https://www.npmjs.org/package/color) module, defaults to black.
  * @returns {Sharp}
  */