--- a/testing/marionette/test_assert.js
+++ b/testing/marionette/test_assert.js
@@ -10,16 +10,18 @@ Cu.import("chrome://marionette/content/a
 Cu.import("chrome://marionette/content/error.js");
 
 add_test(function test_session() {
   assert.session({sessionId: "foo"});
   for (let typ of [null, undefined, ""]) {
     Assert.throws(() => assert.session({sessionId: typ}), InvalidSessionIDError);
   }
 
+  Assert.throws(() => assert.session({sessionId: null}, "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_platforms() {
   // at least one will fail
   let raised;
   for (let fn of [assert.firefox, assert.fennec]) {
     try {
@@ -33,91 +35,110 @@ add_test(function test_platforms() {
   run_next_test();
 });
 
 add_test(function test_noUserPrompt() {
   assert.noUserPrompt(null);
   assert.noUserPrompt(undefined);
   Assert.throws(() => assert.noUserPrompt({}), UnexpectedAlertOpenError);
 
+  Assert.throws(() => assert.noUserPrompt({}, "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_defined() {
   assert.defined({});
   Assert.throws(() => assert.defined(undefined), InvalidArgumentError);
 
+  Assert.throws(() => assert.noUserPrompt({}, "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_number() {
   assert.number(1);
   assert.number(0);
   assert.number(-1);
   assert.number(1.2);
   for (let i of ["foo", "1", {}, [], NaN, Infinity, undefined]) {
     Assert.throws(() => assert.number(i), InvalidArgumentError);
   }
+
+  Assert.throws(() => assert.number("foo", "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_callable() {
   assert.callable(function () {});
   assert.callable(() => {});
 
   for (let typ of [undefined, "", true, {}, []]) {
     Assert.throws(() => assert.callable(typ), InvalidArgumentError);
   }
 
+  Assert.throws(() => assert.callable("foo", "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_integer() {
   assert.integer(1);
   assert.integer(0);
   assert.integer(-1);
   Assert.throws(() => assert.integer("foo"), InvalidArgumentError);
   Assert.throws(() => assert.integer(1.2), InvalidArgumentError);
 
+  Assert.throws(() => assert.integer("foo", "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_positiveInteger() {
   assert.positiveInteger(1);
   assert.positiveInteger(0);
   Assert.throws(() => assert.positiveInteger(-1), InvalidArgumentError);
   Assert.throws(() => assert.positiveInteger("foo"), InvalidArgumentError);
 
+  Assert.throws(() => assert.positiveInteger("foo", "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_boolean() {
   assert.boolean(true);
   assert.boolean(false);
   Assert.throws(() => assert.boolean("false"), InvalidArgumentError);
   Assert.throws(() => assert.boolean(undefined), InvalidArgumentError);
 
+  Assert.throws(() => assert.boolean(undefined, "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_string() {
   assert.string("foo");
   assert.string(`bar`);
   Assert.throws(() => assert.string(42), InvalidArgumentError);
 
+  Assert.throws(() => assert.string(42, "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_window() {
   assert.window({closed: false});
 
   for (let typ of [null, undefined, {closed: true}]) {
     Assert.throws(() => assert.window(typ), NoSuchWindowError);
   }
 
+  Assert.throws(() => assert.window(null, "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_contentBrowser() {
   assert.contentBrowser({contentBrowser: 42, window: {closed: false}});
 
   let closedWindow = {contentBrowser: 42, window: {closed: true}};
   let noContentBrowser = {contentBrowser: null, window: {closed: false}};
@@ -142,29 +163,35 @@ add_test(function test_object() {
 });
 
 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);
   }
 
+  Assert.throws(() => assert.in("foo", {bar: 42}, "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_array() {
   assert.array([]);
   assert.array(new Array());
   Assert.throws(() => assert.array(42), InvalidArgumentError);
   Assert.throws(() => assert.array({}), InvalidArgumentError);
 
+  Assert.throws(() => assert.array(42, "custom"), /custom/);
+
   run_next_test();
 });
 
 add_test(function test_that() {
   equal(1, assert.that(n => n + 1)(1));
   Assert.throws(() => assert.that(() => false)());
   Assert.throws(() => assert.that(val => val)(false));
   Assert.throws(() => assert.that(val => val, "foo", SessionNotCreatedError)(false),
       SessionNotCreatedError);
 
+  Assert.throws(() => assert.that(() => false, "custom")(), /custom/);
+
   run_next_test();
 });