NPE in executable model using an existental pattern in an accumulate and binding the accumulated value with a from

accumulate ( $p: Person ( getName().startsWith(\"M\") ) and exists(String());
  $sum : sum($p.getAge())
)
 $s: String() from $sum.toString()  

java.lang.NullPointerException
	at org.drools.core.rule.LogicTransformer.processElement(LogicTransformer.java:193)
	at org.drools.core.rule.LogicTransformer.processElement(LogicTransformer.java:168)
	at org.drools.core.rule.LogicTransformer.processElement(LogicTransformer.java:250)
	at org.drools.core.rule.LogicTransformer.fixClonedDeclarations(LogicTransformer.java:153)
	at org.drools.core.rule.LogicTransformer.transform(LogicTransformer.java:96)
	at org.drools.core.definitions.rule.impl.RuleImpl.getTransformedLhs(RuleImpl.java:601)
	at org.drools.core.reteoo.builder.ReteooRuleBuilder.addRule(ReteooRuleBuilder.java:115)
	at org.drools.core.reteoo.ReteooBuilder.addRule(ReteooBuilder.java:110)