getting invalid user exception when there was two users selected in picker user and showing as read only in UI as text

Hi 

In Ui there was user picker field if i choose more than one (max selection is 3 ) and submit request after that in read only the selected users we are showing in summary dashboard there user want first name and last name to be displayed instead user name 

Below is the logic i am using to display selected user details in text field

a!textField(
label: "Requested On Behalf:",
labelPosition: "ADJACENT",
value: a!forEach(
items: a!defaultValue(ri!MDSRequest.requestedonBehalf, {}),
expression: if(
a!isNullOrEmpty(fv!item),
{},
if(
a!isNullOrEmpty(touser(fv!item)),
{},
user(touser(fv!item), "firstName") & " " & user(touser(fv!item), "lastName")
)
)
),
readOnly: true,
showWhen: a!isNotNullOrEmpty(ri!MDSRequest.requestedonBehalf)
)
}
),

If i select two means more than one i am getting below error i selected valid users only still getting this error also before upgrade logic worked after version upgrade i am seeing this can any one suggest on this 

 Error Evaluating UI Expression Expression evaluation error [evaluation ID = ef19e:da996] in rule 'changerequestsection_display' (called by rule 'caserecorddashboard') at function a!forEach [line 137]: Error in a!forEach() expression during iteration 1: Expression evaluation error at function 'user' [line 145]: [InvalidUserException] (APNX-1-4198-000)