Why Does Named Parameter For A Custom Attribute Require `:` Syntax?

I defined the following custom attribute:

[AttributeUsage(AttributeTargets.All)]
public class CustomAuthAttribute : AuthorizeAttribute
{
    private MyPermissionLevels requestedAccessLevel;

    public CustomAuthAttribute(object AccessLevel = null)
    {
        AccessLevel = AccessLevel ?? MyPermissionLevels.View;            
        if(AccessLevel.GetType() != typeof(MyPermissionLevels))
        {
            throw new ArgumentException("AccessLevel is invalid.");
        }
        this.requestedAccessLevel = (MyPermissionLevels)AccessLevel;
    }

        ... // Other Auth Stuff
}

When I apply this attribute to a controller as, for example, [CustomAuthAttribute (AccessLevel = MyPermissionLevels.Modify)] intellisense complains that:

The type or namespace name 'AccessLevel' could not be found. (etc?)

The potential fixes revealed by Alt + Enter include:

Change 'AccessLevel' to 'AccessLevel:'

Adding this colon (and by implication removing the = operator, which is not an automagical part of the suggested fix) does resolve the compilation error.

I am okay using a different syntax if that's all that need be. However, I would like to understand what I've done that prevents me from using the more common attribute field-initialization syntax as described in answers to Named parameters confusion and as can be done with the parent class:

[Authorize(Roles = "Modifiers")]