Referencing a controller extension that has a namespace

Hi,

I'm trying to create a visualforce page and reference an extension that is located in a managed package and has a namespace "nsp". Note: the visualforce page is not in the managed package. The visualforce page looks like this:

<apex:page standardController="Account" extensions="nsp__testextension">

    
</apex:page>

However when I try to save the above the error is "Error: The name can only contain underscores and alphanumeric characters. It must begin with a letter and be unique, and must not include spaces, end with an underscore, or contain two consecutive underscores."

The controller extension looks like this:

global class ControllerExtension{
    
    global ControllerExtension(ApexPages.StandardController stdController) {
    
    }


    @RemoteAction
    global static String helloWorld(String recordId) {
    
        return 'Record id in context: '+recordId;
    
    }
    
}

Are extensions with namespace supported?