title: R "Error in density.default(raw) : argument 'x' must be numeric" in data.frame with all int values
body: <p>Why am I getting this error when all of my types seem to be numeric?</p>
  
  <pre><code>&gt; str(raw)
  'data.frame':   404166147 obs. of  8 variables:
   $ piece_1_A  : int  0 0 0 0 0 0 0 0 0 0 ...
   $ piece_1_B  : int  0 0 0 0 0 0 0 0 0 0 ...
   $ piece_2_A : int  0 0 0 0 0 0 0 0 0 0 ...
   $ piece_2_B : int  0 0 0 0 0 0 0 0 0 0 ...
   $ item_1_A : int  0 0 0 0 0 0 0 0 0 0 ...
   $ item_1_B : int  0 0 0 0 0 0 0 0 0 0 ...
   $ piece_2_A: int  0 0 0 0 0 0 0 0 0 0 ...
   $ piece_2_B: int  0 0 0 0 0 0 0 0 0 0 ...
  &gt; sapply(raw, typeof)
    item_1_A   item_1_B  item_2_A  item_2_B  piece_1_A  piece_1_B piece_2_A
     "integer"    "integer"    "integer"    "integer"    "integer"    "integer"    "integer" 
  piece_2_B 
     "integer" 
  &gt; density(raw[,1])
  Error in density.default(raw[, 1]) : argument 'x' must be numeric
  </code></pre>