title: "AttributeError: sqrt" when calculating a simple standard deviation
body: <p>I found a very unusual error when trying to calculate the standard deviation of a two dimensional numpy array. Basically, I'm doing this:</p>
  
  <pre><code>np.std(myarray, axis=1)
  </code></pre>
  
  <p>which gives the following error:</p>
  
  <pre><code>/home/user/env/local/lib/python2.7/site-packages/numpy/core/fromnumeric.pyc in std(a, axis, dtype, out, ddof, keepdims)
     2588 
     2589     return _methods._std(a, axis=axis, dtype=dtype, out=out, ddof=ddof,
  -&gt; 2590                                 keepdims=keepdims)
     2591 
     2592 def var(a, axis=None, dtype=None, out=None, ddof=0,
  
  /home/user/env/local/lib/python2.7/site-packages/numpy/core/_methods.pyc in _std(a, axis, dtype, out, ddof, keepdims)
      103 
      104     if isinstance(ret, mu.ndarray):
  --&gt; 105         ret = um.sqrt(ret, out=ret)
      106     else:
      107         ret = um.sqrt(ret)
  
  AttributeError: sqrt
  </code></pre>
  
  <p>In the line 105, ret is defined as:</p>
  
  <pre><code>array([0.0757800982464383, 0.6065241443345735, 0.3162436337971689,
         0.025387106329804794, 0.023465650294750118, 0.01234409423996419,
         0.03686346121524665, 0.456152653196993, 0.15598749370862977,
         0.0041977155187445945, 0.018816207536006213, 0.018011541017004237,
         0.01046808236307669, 0.0037176987848958156, 0.004346127061033225,
         0.06885161954332783, 0.004758430435294487, 0.010064124660786879,
         0.08732648466448349, 0.14957009536890314, 0.007277246755033778,
         0.0043521569980290355, 0.010174973078043143, 0.33905025844712544,
         0.7960121881423348], dtype=object)
  </code></pre>
  
  <p><code>type(myarray): &lt;type 'numpy.ndarray'&gt;</code></p>
  
  <pre><code>repr(myarray): array([[1.2258313, 1.2258313, 1.3756552, 1.1849703, 1.334794, 1.1849703,
              1.1441092, 1.334794, 1.3075534, 1.2258313, 1.3756552, 0.95342433,
              1.1441092, 1.0760075, 1.1168685, 1.1168685, 1.334794, 0.8036005,
              0.46309182, 0.3405087],
             [1.3756552, 0.95342433, 1.1441092, 1.0760075, 1.1168685, 1.1168685,
              1.334794, 0.8036005, 0.46309182, 0.3405087, 0.313268, 0.38136974,
              0.27240697, 0.38136974, -1.8387468999999996, -0.50395286,
              -0.14982383, -0.46309182, -0.3405087, -0.19068487],...
  </code></pre>
  
  <p>buy I can't see anything wrong with that array. np.sum and np.mean work correctly.</p>
  
  <p>What could be the cause of this error?</p>