Codeigniter unable to convert number to words

Hi iam a new devloper on codeigniter i was loking for a libraries to convert numbers to words and i have found this : https://gist.github.com/opnchaudhary/4721977

i have put numbertowords.php on /application/libraries folder

this is my code :

        

          

            
              : 
            numbertowords->convert_number($row['amount']); ?>
            
          
        
      </div>

this is the error:

Severity: Error --> Call to a member function convert_number() on null

Please help me to resolve this