Jan-Willem van den Broek reports:

grammar TestPredicate;

a : v=vv '=' ( {$v.text.equals('v')}?=> 'c' | 'd');

vv : 'v';

Here is the error:

error(10): internal error: C:\Documents and Settings\jwbroek\My Documents\antlr_test\TestPredicate.g : java.util.NoSuchElementException: no such attribute: TREE_PARSER in template context [evalPredicate ruleLabelPropertyRef_text]
org.antlr.stringtemplate.StringTemplate.checkNullAttributeAgainstFormalArguments(StringTemplate.java:1290)
org.antlr.stringtemplate.StringTemplate.get(StringTemplate.java:814)