New-SPAlternateURL : You must specify the default zone URL for all Web applications ...

I was configuring AAM using Powershell. I wanted my current Default URL to be my new Intranet URL and to use a new URL as my Default. To add a setting for a new zone, I was using New-SPAlternateURL. 


$app = Get-SPWebApplication
New-SPAlternateURL -WebApplication $app -Zone "Intranet" -Url $app.Url
...

However, this is what I am getting:

New-SPAlternateURL : You must specify the default zone URL for all Web applications. To delete the Alternate URL Collection, remove the public URLs in all other zones, and then remove the default zone URL


It turns out this is the error message you get when you try to use the same URL for more than one zone. It would have been nice if the error message actually said that. So now my code is like this:


$app = Get-SPWebApplication
$intranetUrl = $app.Url
Set-SPAlternateURL -Identity $app.Url -Zone "Default" -Url $defaultUrl
New-SPAlternateURL -WebApplication $app -Zone "Intranet" -Url $intranetUrl