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What does the  "a._known.slice is not a function" error message mean?

Question asked by dcrsGIS_frod on Aug 12, 2020
Latest reply on Aug 12, 2020 by xander_bakker

I have previously used this Arcade expression with other datasets and haven not had any issues, I tried running this expression today: 


var portal = Portal('');
var features = FeatureSetByPortalItem(portal, 'ed174bb08dc34850ac8d0e719cae0c72', 0, ['*'], true);

var open = $feature;
var landid = contains(open, features);

var cnt = Count(landid);

var resultado = "This Open-Space parcel contains " + cnt+ " "+ "Land ID Points";
for (var f in landid) {
resultado += TextFormatting.NewLine + "Land Id(s):" + text(

return resultado

When I test the expression out in the webmap, I get the expected results:


Result :Value
Result: This Open-Space parcel contains 1 Land ID Points
Land Id(s):10196
Type: String


However, when I try running the expression using  the field calculator on a string field (255 characters) in the $feature, I keep getting this error message, 


a._known.slice is not a function


Can anyone point me in the right direction as to what I am doing wrong with my syntax?