title: How to define the default missing value in pandas dataframe
body: <p>I want to read a dataframe with given datatype and missing values, but the follwing code is wrong. I have no idea, why this happens!</p>
  
  <pre><code>myText = StringIO("""1,2
  3,\N
  5,6""")
  
  myDf = pd.read_csv(myText, header=None, names=["a1","a2"], na_values=["\N"], dtype={"a1":"int", "a2":"int"})
  </code></pre>
  
  <p>I got the error message:</p>
  
  <pre><code>ValueError: Integer column has NA values in column 1
  </code></pre>
  
  <p>If I remove the dtype option <code>dtype={"a1":"int", "a2":"int"}</code>,  then it works fine. Does the integer column don't allow missing values?</p>