antlr: Can't find nondeterminism in simple parser

Hi!
Can somebody please help me finding out why antlr complains about this
grammar?

list : key (WHITESPACE)+ value ((WHITESPACE)+ key (WHITESPACE)+
value)* ;

value :  integer  | LEFTBRACKET (WHITESPACE)* list (WHITESPACE)*
RIGHTBRACKET;

key : LETTER (LETTER|DIGIT)*;

integer : (DIGIT)+;

The lexer-specification is straightforward:

WHITESPACE : (' ' | '\t' | '\r' '\n' | '\n');
LETTER : ('a'..'z'|'A'..'Z');
DIGIT : '0'..'9';
LEFTBRACKET  : '[';
RIGHTBRACKET : ']';

And this is the error message:

gml2.g:4: warning:nondeterminism upon
gml2.g:4:     k==1:WHITESPACE
gml2.g:4:     k==2:WHITESPACE
gml2.g:4:     k==3:WHITESPACE,LETTER
gml2.g:4:     k==4:WHITESPACE,LETTER,DIGIT
gml2.g:4:     k==5:WHITESPACE,LEFTBRACKET,RIGHTBRACKET,LETTER,DIGIT
gml2.g:4:     between alt 1 and exit branch of block

It disappears when I delete the second (WHITESPACE)* in the value
rule. But that is not an option because the files which should be
parsed may be formatted with whitespace chars.
I am really stuck with this.

Thank you very much in advance
Olaf