diff --git a/lib/type/matrix/Spa.js b/lib/type/matrix/Spa.js
index 70f652ebe..72ff0bf92 100644
--- a/lib/type/matrix/Spa.js
+++ b/lib/type/matrix/Spa.js
@@ -20,6 +20,12 @@ function factory (type, config, load, typed) {
     this._heap = new type.FibonacciHeap();
   }
   
+  /**
+   * Set the value for index i.
+   *
+   * @param {Number} i                       The index
+   * @param {Number | BigNumber | Complex}   The value at index i
+   */
   Spa.prototype.set = function (i, v) {
     // check we have a value @ i
     if (!this._values[i]) {