title: ISNUMBER() validation rule for numbers
body: <p>I am trying to add a new validation rule <code>ISNUMBER(number)</code> on a number type field.</p>
  
  <p>but it seems like I can't because I got a syntax error, <code>Error: Incorrect argument type for function 'ISNUMBER()'</code></p>
  
  <p>How could I validate this field and check if it's a number ?</p>