[antlr-interest] strange MismatchedTokenException

[Simon zack]

hi,

I'm new to antler
tried writing a little grammer, but it tells me MismatchedTokenException
when I interpret namespace_specification
using the string:
"hello.world"

[code]
grammar test;

exception_info
    :
        '.exception'
    ;

namespace_specification
    :
        ns_id1 = IDENTIFIER '.'
    ;

IDENTIFIER
    :    LETTER (LETTER|'0'..'9')*
    ;

LETTER
    :    '$'
    |    'A'..'Z'
    |    'a'..'z'
    |    '_'
    ;

[/code]

what am I doing wrong?
thanks