title: Error in fix.by(by.x, x) : 'by' must specify a uniquely valid column
body: <p>I have just installed the latest version (0.98.501) of RStudio on Windows 7.</p>
  
  <p>I am trying to project two columns of a table with the following call.</p>
  
  <pre><code>total &lt;- merge(dayTable$name1,dayTable$name2,by="ID")
  </code></pre>
  
  <p>This results in the following error</p>
  
  <pre><code>Error in fix.by(by.x, x) : 'by' must specify a uniquely valid column
  </code></pre>
  
  <p>I tried typing in</p>
  
  <pre><code>dayTable$name1
  dayTable$name2
  </code></pre>
  
  <p>and got two valid integer vectors of the same length.</p>