Error using photons with a MergingTransformer

When I try use make_photons() in HLTParticleFlowRun3 (MergingTransformer), I get the error bellow (last lines):

File "/afs/cern.ch/work/r/rangel/lhcb/rta/stack_20191202/Moore/PyConf/python/PyConf/components.py", line 73, in _json_dump raise TypeError(repr(obj) + " is not json serializable") TypeError: DataHandle('/Event/FunctionalParticleMakerBest/Output') is not json serializable

any other particle inputs work, like make_phi2kk().