diff --git a/assert.js b/assert.js
index a9aa6ad..4e89129 100644
--- a/assert.js
+++ b/assert.js
@@ -130,6 +130,9 @@ assert.AssertionError = function AssertionError(options) {
   this.expected = options.expected;
   this.operator = options.operator;
   if (options.message) {
+    if (typeof options.message === 'function') {
+      options.message = options.message();
+    }
     this.message = options.message;
     this.generatedMessage = false;
   } else {