title: Merge r brings error "'by' must specify uniquely valid columns"
body: <p>R doesn't like me today...</p>
  
  <p>I have two tables asembled via cbind(). Tab 1 (<strong>dwd_nogap</strong>) is</p>
  
  <pre><code>  x1                 col1_x1       col2_x1      
  A "1982 12 01 00:00" "        0.4" "          0"
  B "1982 12 02 00:00" "       -0.5" "          0"
  C "1982 12 03 00:00" "       -0.2" "          0"
  D "1982 12 04 00:00" "         -1" "        0.1"
  E "1982 12 05 00:00" "       -0.9" "          0"
  F "1982 12 06 00:00" "        3.7" "        4.1"
  </code></pre>
  
  <p>Tab 2 (<strong>dwd_gap</strong>) is:</p>
  
  <pre><code>     x2                 col1_x2       col2_x2      
  [1,] "1982 12 01 00:00" "        0.4" "          0"
  [2,] "1982 12 03 00:00" "       -0.2" "          0"
  [3,] "1982 12 04 00:00" "         -1" "        0.1"
  [4,] "1982 12 05 00:00" "       -0.9" "          0"
  [5,] "1982 12 06 00:00" "        3.7" "        4.1"
  [6,] "1982 12 07 00:00" "          7" "        5.8"
  </code></pre>
  
  <p>My merge command is:</p>
  
  <pre><code>exporttab &lt;- merge(x=dwd_nogap,y=dwd_gap,by.x=dwd_nogap[,1],by.y=dwd_gap[,1], fill=-9999)
  </code></pre>
  
  <p>In my opinion the command is correct, but it's apparently not doing well...</p>
  
  <pre><code>Error in fix.by(by.x, x) : 'by' must specify uniquely valid columns
  </code></pre>