I "solved" the problem shortly after starting this thread. Specifying a land cost of $0 gives the correct (long-term gain) result. Leaving the land cost box empty results in the same gain but classified as short-term. I forgot to check the "solved" box until today.
In my particular case, the property came to my wife by inheritance via a trust which eventually dissolved. At the time of dissolution, the trustee provided only the cost basis for the building. The land cost was not specified and wasn't needed until she sold the property. We worked out the land cost by subtracting the building basis from the total value declared in her mother's estate tax return.
A zero cost basis for land would be possible if a real property had be acquired for free under the Homestead act and then passed down by gift during the lifetimes of each previous owner. Not a very common situation.
