MASSWITHHYPOTHESES functor crashes

When trying to use the MASSWITHHYPOTHESES functor, Moore crashes with a long stack trace. This is the first error I see:

/Rec/InstallArea/x86_64_v2-centos7-gcc10-opt/include/Functors/Composite.h:321:53: error: no member named 'numChildren' in 'LHCb::Particle'
        if ( sizeof...( MassInputs ) != combination.numChildren() ) {

I'm calling F.MASSWITHHYPOTHESES(["pi+", "pi-"]) in the CompositeCut of the ThOr ParticleCombiner. The DecayDescriptor is [Lambda0 -> p+ pi-]cc.