title: Mysql 5.7 st_distance_sphere return every time null
body: <p>this is my code:</p>
  
  <pre><code>SELECT st_distance_sphere( POINT(8,51) ,  point(@lon, @lat)) as distance FROM places;
  </code></pre>
  
  <p>If I execute it, all rows returns null. </p>
  
  <p>Maybe the @lon @lat doesnt function? 
  I don´t know... </p>
  
  <p>If I replace the @lon @lat with for example 9 and 51 , it return '69976.54452205829' meters... </p>