title: Error in strsplit(x, split = "") : non-character argument
body: <p>I'm trying to use <code>protr</code> package in r but I got error when I tried to extract amino acid composition  from the sequence as the following:</p>
  
  <pre><code>require(protr) 
  x = readFASTA(system.file('protseq/P00750.fasta',   package='protr'))
  extractAAC(x)
  # Error in strsplit(x, split = "") : non-character argument
  </code></pre>
  
  <p>how can I fix this error?</p>