--- a/testing/marionette/assert.js
+++ b/testing/marionette/assert.js
@@ -298,17 +298,17 @@ assert.string = function (obj, msg = "")
  */
 assert.object = function (obj, msg = "") {
   msg = msg || error.pprint`Expected ${obj} to be an object`;
   return assert.that(o => {
     // unable to use instanceof because LHS and RHS may come from
     // different globals
     let s = Object.prototype.toString.call(o);
     return s == "[object Object]" || s == "[object nsJSIID]";
-  })(obj);
+  }, msg)(obj);
 };
 
 /**
  * Asserts that |prop| is in |obj|.
  *
  * @param {?} prop
  *     Own property to test if is in |obj|.
  * @param {?} obj

--- a/testing/marionette/test_assert.js
+++ b/testing/marionette/test_assert.js
@@ -131,16 +131,18 @@ add_test(function test_contentBrowser() 
 
 add_test(function test_object() {
   assert.object({});
   assert.object(new Object());
   for (let typ of [42, "foo", true, null, undefined]) {
     Assert.throws(() => assert.object(typ), InvalidArgumentError);
   }
 
+  Assert.throws(() => assert.object(null, "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_in() {
   assert.in("foo", {foo: 42});
   for (let typ of [{}, 42, true, null, undefined]) {
     Assert.throws(() => assert.in("foo", typ), InvalidArgumentError);
   }