diff --git a/lib/test.js b/lib/test.js
index ea139cb4a..990d5ca1a 100644
--- a/lib/test.js
+++ b/lib/test.js
@@ -613,7 +613,7 @@ class Test extends Base {
     if (Test.prototype[name] || this[name])
       throw new TypeError('attempt to re-define `' + name + '` assert')
 
-    const ASSERT = (...args) => {
+    const ASSERT = function (...args) {
       this.currentAssert = ASSERT
       if (typeof args[length] === 'object') {
         args[length + 1] = args[length]
diff --git a/test/test.js b/test/test.js
index c52fa8107..7bad194e5 100644
--- a/test/test.js
+++ b/test/test.js
@@ -1031,6 +1031,15 @@ t.test('addAssert', t => {
   return t.end()
 })
 
+t.test('addAssert on prototype', t => {
+  function foobar (found, message, extra) {
+    return this.ok(found, message, extra)
+  }
+  Test.prototype.addAssert('foobar', 1, foobar)
+  t.foobar(true, 'this is fine')
+  t.end()
+})
+
 t.test('spawn', t => {
   const okjs = path.resolve(__dirname, '../ok.test.js')
   t.teardown(() => fs.unlinkSync(okjs))