Pandas Panel problem

gistfile1.py

import pandas

wp = pandas.Panel(randn(2, 5, 4), items=['Item1', 'Item2'],
                    major_axis=pandas.date_range('1/1/2000', periods=5),
                    minor_axis=['A', 'B', 'C', 'D'])
wp2 = pandas.Panel(randn(3, 5, 4), items=['Item1', 'Item2', 'Item3'],
                    major_axis=pandas.date_range('1/1/2000', periods=5),
                    minor_axis=['A', 'B', 'C', 'D'])

wp.sub(wp2).ix[:, :, 'A']

# Output:
#               Item1     Item2     Item3
# 2000-01-01  1.182223  0.385267 -0.370692
# 2000-01-02 -0.358195 -0.040343  0.350876
# 2000-01-03 -0.237364  0.213128  0.313635
# 2000-01-04 -1.093256  0.620667  1.686411
# 2000-01-05 -1.906565 -1.295648 -0.644578
#
# Why is column 'Item3' there?

This actually seems to be particular to the items axis, the following seems to work:

(wp.transpose(1,0,2)).sub(wp2.transpose(1,0,2)).ix[:, :, 'A'].T

               Item1     Item2  Item3
2000-01-01  0.398772 -0.036486    NaN
2000-01-02 -0.386096  1.152324    NaN
2000-01-03 -2.046829  0.582434    NaN
2000-01-04  0.137687 -0.661586    NaN
2000-01-05  0.480794 -0.362363    NaN

Pandas version is 0.12.0