extending nlobjRecord = "nlobjRecord is not defined"

I'm experimenting with extending the nlobjRecord class with some additional properties.

My first attempt:

nlobjRecord.prototype.isDirty = false;

fails with the error "nlobjRecord is not defined"

Just for kicks I tried nlobjSearchFilter (I know it's defined because I'm creating them) and it works.  Same code, different object:

nlobjSearchFilter.prototype.isDirty = false;

Snooping around the nlapi source doesn't clear anything up, both classes are defined in the same file with the same syntax.

So I'm really confused by this, hoping that a javascript guru can explain how this is possible.

Here is the entire anonymous method that I have at the top of my script: