Is this an C# 4.0 compiler optional parameters bug?

I'm writing custom security attribute and got strange compiler behaviour... When I'm using the attribute at the same file, default parameter values works fine:

using System.Security.Permissions;

[System.Serializable]
sealed class FooAttribute : CodeAccessSecurityAttribute {
    public FooAttribute(SecurityAction action = SecurityAction.Demand) : base(action) { }
    public override System.Security.IPermission CreatePermission() { return null; }
}

[Foo] class Program {
    static void Main(string[] args) { }
}

But when I'm separating the code above into two files like that - file 1:

using System.Security.Permissions;

[System.Serializable]
sealed class FooAttribute : CodeAccessSecurityAttribute {
    public FooAttribute(SecurityAction action = SecurityAction.Demand) : base(action) { }
    public override System.Security.IPermission CreatePermission() { return null; }
}

And file 2:

[Foo] class Program {
    static void Main(string[] args) { }
}

I've got an compiler error:

Error: 'FooAttribute' does not contain a constructor that takes 0 arguments

This occurs only with the CodeAccessSecurityAttribute inheritors, looks very strange...