WebStyle = Nil <> _DefaultStyle?

In my web project there is a WebTextField named txtPharmacy and no WebStyle is used (Style = _DefaultStyle).
Input in that field is required.
When txtPharmacy is empty, a red border is drawn around the WebTextField by using a WebStyle named stlInvalid.
As soon as data is entered in the field, the WebStyle needs to be set back to _DefaultStyle.

 txtPharmacy.Style = Nil

I expected the WebStyle to be _DefaultStyle again, but the WebStyle is now the non existing txtPharmacy_Style.
I thought that setting the WebStyle to Nil would make it _DefaultStyle again.
Apparently not, what is the correct way to set a WebStyle to _DefaultStyle or is this a bug?

Problem ‘solved’ by using a workaround.
I created a WebStyle stlDefault that has no properties.

txtPharmacy.Style = stlDefault gives me the same result as _DefaultStyle.