Cannot compile rules that use functions within collects

import java.util.*;

global java.util.Set controlSet;
global org.example.drools.service.DummyService dummyService;

function String defaultIfBlank(String input, String default){
 return "";
}

rule "will execute per each Measurement having ID color"
no-loop
when
 Measurement( id == "color", $colorVal : val )
 List(size() > 0) from accumulate(String($s: this) from dummyService.dummy($colorVal), collectList(defaultIfBlank($s, "test")))
then
 controlSet.add($colorVal);
end

[ERROR] Message [id=1, kieBase=defaultKieBase, level=ERROR, path=rules/dummy-rule.drl, line=-1, column=0
[ERROR] text=Root node of expression should be a declaration]