[antlr-interest] Simple grammar with error

[Alfonso]

I'm using ANTLRWorks to learn the basics of ANTLR. Can someone explain 
me why this grammar


grammar Failed;

expression
    : term ( (PLUS|MINUS) expression)?
    EOF
    ;
 
term    :
      atom ( (DIVIDE | ASTERISK) term)?
    ;

atom    :
      NUMBER
    | OPEN_PAREN expression CLOSE_PAREN
    ;

NUMBER    : '0'..'9' ('0'..'9')*;
   
PLUS     : '+';
MINUS    : '-';
DIVIDE  : '/';
ASTERISK : '*';

OPEN_PAREN : '(' ;
CLOSE_PAREN: ')' ;

Generates a "NoViableAltException" after the "5" when facing the 
expression 2*(3+5)+1 in the interpreter?

Surely this is a basic question, but for god's sake I'm not able to 
understand it.

Thanks in advance