Hi, I will close this issue in the pharo tracker: 13246 MNU RBArgumentNode>>key with parameterized Seaside 3.1 REST Method https://pharo.fogbugz.com/f/cases/13246 the solution is to change #argumentNamesOf: to be: argumentNamesOf: aCompiledMethod ^ aCompiledMethod argumentNames (works of Pharo3 and Pharo4) Marcus