ubuntu 12.04: test_nanargmin failure
```
======================================================================
ERROR: test_nanargmin (pandas.tests.test_nanops.TestnanopsDataFrame)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/buildd/pandas-0.14.0+git213-g741b2fa/debian/tmp/usr/lib/python2.7/dist-packages/pandas/tests/test_nanops.py", line 346, in test_nanargmin
    allow_str=False, allow_obj=False)
  File "/tmp/buildd/pandas-0.14.0+git213-g741b2fa/debian/tmp/usr/lib/python2.7/dist-packages/pandas/tests/test_nanops.py", line 240, in check_funs
    self.check_fun(testfunc, targfunc, 'arr_date', **kwargs)
  File "/tmp/buildd/pandas-0.14.0+git213-g741b2fa/debian/tmp/usr/lib/python2.7/dist-packages/pandas/tests/test_nanops.py", line 202, in check_fun
    testarval, targarval, targarnanval, **kwargs)
  File "/tmp/buildd/pandas-0.14.0+git213-g741b2fa/debian/tmp/usr/lib/python2.7/dist-packages/pandas/tests/test_nanops.py", line 158, in check_fun_data
    targ = targfunc(targartempval, axis=axis, **kwargs)
  File "/tmp/buildd/pandas-0.14.0+git213-g741b2fa/debian/tmp/usr/lib/python2.7/dist-packages/pandas/tests/test_nanops.py", line 323, in _argminmax_wrap
    res = func(value, axis)
  File "/usr/lib/python2.7/dist-packages/numpy/core/fromnumeric.py", line 754, in argmin
    return argmin(axis)
TypeError: ("ufunc 'subtract' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule 'safe'", 'axis: 0 of 2', 'skipna: False', 'kwargs: {}', 'testar: arr_date', 'targar: arr_date', 'targarnan: arr_date')

```

that was while building 0.14.0+git213-g741b2fa 

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.2.0-4-amd64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: C
LANG: C

pandas: 0.14.0.dev
nose: 1.1.2
Cython: 0.15.1
numpy: 1.6.1
scipy: 0.9.0
statsmodels: 0.5.0
IPython: None
sphinx: 1.1.3
patsy: 0.2.1
scikits.timeseries: None
dateutil: 1.5
pytz: 2012c
bottleneck: None
tables: 2.3.1
numexpr: 1.4.2
matplotlib: 1.1.1rc
openpyxl: 1.7.0
xlrd: 0.6.1
xlwt: 0.7.2
xlsxwriter: None
lxml: None
bs4: 4.0.2
html5lib: 0.90
bq: None
apiclient: None
rpy2: None
sqlalchemy: None
pymysql: None
psycopg2: None
```
