asks to put a ")" and not at end

Ok, here's my program:

var i: integer;

type string10 = string[10];

const
  ones: array[0..9] of string10 = ('','one','two','three','four','five','six','seven','eight','nine');
  tens: array[2..9] of string10 = ('','twenty','thirty','forty','fifty','sixty','seventy','eighty','ninety');


function wordnumber (number: word) : string;
  begin
     wordnumber:= tens [number div 10] + '-' +ones [number div 10];
  end;

begin {main}
   for i:= 20 to 99 do
       writeln ('You write ' i, ' like this: ', wordnumber(i));
end.

Why does it ask me to put a "(" at the comma after the 'eighty'? It's not at the end of the line.