diff --git a/lib/unexpected.js b/lib/unexpected.js
index <HASH>..<HASH> 100644
--- a/lib/unexpected.js
+++ b/lib/unexpected.js
@@ -632,7 +632,7 @@ var root = this;
         expect(this.obj, 'to be a map whose values satisfy', callback);
     });
 
-    expect.addAssertion('to be (an|a) [non-empty] (map|hash|object) whose keys satisfy', function (callback) {
+    expect.addAssertion('to be (a|an) [non-empty] (map|hash|object) whose keys satisfy', function (callback) {
         if ('function' !== typeof callback) {
             throw new Error('Assertions "' + this.testDescription + '" expects a functions as argument');
         }
Put (a|an) in a consistent order.