NoMatchingRootException Espresso with custom Toast

I'm starting out with Espresso and I've got a custom toast that I show when the user puts in something wrong (I know I can use EditText.setError(), but that's just how it is).

Here's the code for the Toast

 private static void showToast(Context context, String message, boolean isError) {
        if (context != null) {
            LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            TextView toastView = (TextView) inflater.inflate(R.layout.error_toast_layout, null);
            if (!isError) {
                toastView.setBackgroundColor(context.getResources().getColor(R.color.alert_positive));
            }
            toastView.setText(message);
            Toast errorToast = new Toast(context);
            errorToast.setGravity(Gravity.TOP, 0, 0);
            errorToast.setDuration(Toast.LENGTH_SHORT);
            errorToast.setView(toastView);
            errorToast.show();
        }
    }

and I am writing up a little UI test that will assert that correct Toasts are being shown when the user puts in wrong input

Here's the test

   @RunWith(AndroidJUnit4.class)
    @LargeTest
    public class ForgotPasswordTest extends BaseEspressoTest<ForgotPasswordActivity> {

        @Test
        public void testEmailNotEntered() {
            onView(withId(R.id.email_et)).perform(typeText("w"), closeSoftKeyboard());
            onView(withId(R.id.btn_submit)).perform(click());
            onView(withText(R.string.incorrect_email_dialog)).inRoot(withDecorView(not(mActivityRule.getActivity().getWindow().getDecorView()))).check(matches(isDisplayed()));     
    }
}

this works fine if I substitute my custom Toast for a regular Toast, but fails when I try it with mine. I can see that the Toast is being shown during the test.

Here's the error:

android.support.test.espresso.NoMatchingRootException: Matcher 'with decor view not ' did not match any of the following roots: [Root{application-window-token=android.view.ViewRootImpl$W@2d629b84, window-token=android.view.ViewRootImpl$W@2d629b84, has-window-focus=true, layout-params-type=1, layout-params-string=WM.LayoutParams{(0,0)(fillxfill) sim=#22 ty=1 fl=#8d810100 pfl=0x8 wanim=0x1030461 surfaceInsets=Rect(0, 0 - 0, 0)}, decor-view-string=DecorView{id=-1, visibility=VISIBLE, width=1080, height=1920, has-focus=true, has-focusable=true, has-window-focus=true, is-clickable=false, is-enabled=true, is-focused=false, is-focusable=false, is-layout-requested=false, is-selected=false, root-is-layout-requested=false, has-input-connection=false, x=0.0, y=0.0, child-count=1}}]

I've also tried to match the the Toast based on the fact that it's supposed to focus:

onView(withText(R.string.incorrect_email_dialog)).inRoot(isFocusable()).check(matches(isDisplayed()));

but that just says that it can't find it within the view hierarchy:

No views in hierarchy found matching: with string from resource id: <2131165635>[incorrect_email_dialog] value: Please enter a valid email