Store the values of a multipledropdownfield

Help please 

I want to store a multipledropdownfield into my cloud database 

I add the multipledropdownfield, if i pick only value, it works great (see the image bellow)

If i pick or select TWO or more values, an ERROR appears (see the image bellow)

This is the expression of the multipleDropdownField

{
a!multipleDropdownField(
label: "Acceso a servicios basicos?",
labelPosition: "ABOVE",
placeholder: "--- Seleccione ---",
choiceLabels: {
"Agua",
"Luz",
"Teléfono",
"Internet"
},
choiceValues: {
"Agua",
"Luz",
"Teléfono",
"Internet"
},
value: ri!CWA_OECClient.coecaccesoservicionbasicos,
saveInto: ri!CWA_OECClient.coecaccesoservicionbasicos,
required: true,
validations: {}
)}