Using SetColumns with a custom property

I want to improve the speed of iterating through Items using the SetColumns
method, but I run into problems when I try to include a custom property in
the column list. I've used this same custom property in restrict filters,
and have double checked that it is a property of the folder as well as the
items. The custom property is a string type.

The offending line of code is:
Items.SetColumns("MyCustomProperty, FullName, BusinessAddressState")

With the error:
"The Property "MyCustomProperty" is unknown."

It works fine without the custom property in there.


Thanks for any help!