Arcade Expression popup polylines contained within polygon

[StuartMoore]

any ideas why the below will work against an AGOL hosted feature service but not against one hosted from our local ArcServer 

var MLay = Distinct(Contains(FeatureSetByName($map,"lineA"),$feature),'FPERSON')

var ML_array = []
var ML_pop2 = ''

for (var i in MLay ){
  Insert(ML_array,0,i.FPERSON)
}

var ML = Distinct(ML_array)
for (var j in ML){
  ML_pop2 += ML[j] + ", "
}

return "ML(s): " + ML_pop2

[StuartMoore · Accepted solution]

for (var i in MLay ){
  Insert(ML_array,0,i.FPERSON)
}

var c = 0
for (var i in MLay ){
  ML_array[c] = i.FPERSON
  c +=1
}

fixed it with this