Roslyn INamedTypeSymbol.ContainingNamespace.Name an empty string

Question

I have an attribute I'm generating that contains a couple of constructor arguments, a string and a Type, and I'd really like to get the namespace of the Type, because I need to include that in the generated source when using the actual Type.

The attribute is like:

public MyCustomAttribute : Attribute
{
    public MyCustomAttribute(string info, Type type)
    {
        //...
    }
}

My Roslyn code where I'm struggling is:

private void Foo(SemanticModel model, SyntaxNode node)
{
    if(model.GtDeclaredSymbol(node) is not INamedTypeSymbol symbol)
    {
        return null;
    }

    var attributeData = symbol.GetAttributes().First(x => AttributeClass.Name == "MyCustomAttribute");
   
    var stringdata = (string)attributeData.ConstructorArguments[0].Value; //this works fine
    var typedata = (INamedTypeSymbol)attributeData.ConstructorArguments[1].Value;

    var typeName = typedata.MetadataName; // works to get the actual type name
    var typeNamespace = typedata.ContainingNamespace.Name; // This is an empty string?

    //...
}