The following 3 commands:

_screen.AddProperty("PrintScr")    && add the property (no error)
? PEMSTATUS(_Screen,'Printscr',4)  && returns .T.
? PEMSTATUS(_Screen,'xPrintscr',4) && gives error re invalid argument

Interestingly the command following run immediately after returns .F.

? PEMSTATUS(_Screen,'xPrintscr',5)

While it may be (don't know) that a property added by AddProperty is not considered to be "user defined", I don't think that using "4" should generate an error.

This occurred in both VFP8 and VFP7SP1. A search of (online) MSKB using "Visual FoxPro 7" and "pemstatus" returned nothing.