Null argument not allowed

Hi team,

I am  going through this appian tutorial  Process Modeling Tutorial - Appian 21.2  but error in expenseReportSummary Interface like this "Could not display interface. Please check definition and inputs. Interface Definition: Expression evaluation error at function 'user' [line 5]: Null argument not allowed".

i have interface

{
a!textField(
label: "Requester",
labelPosition: "ADJACENT",
value: user(ri!expenseReport['recordType!{618f143c-4310-4256-a9c3-b96fe7161fcf}AT Expense Report.fields.{pp}pp.{initiator}initiator'], "firstName") & " " & user(ri!expenseReport['recordType!{618f143c-4310-4256-a9c3-b96fe7161fcf}AT Expense Report.fields.{pp}pp.{initiator}initiator'],"lastName")
),
a!textField(
label: "Amount",
labelPosition: "ADJACENT",
value: ri!expenseReport['recordType!{618f143c-4310-4256-a9c3-b96fe7161fcf}AT Expense Report.fields.{expenseAmount}expenseAmount'],
readOnly: true
),
a!textField(
label: "Date",
labelPosition: "ADJACENT",
value: ri!expenseReport['recordType!{618f143c-4310-4256-a9c3-b96fe7161fcf}AT Expense Report.fields.{expenseDate}expenseDate'],
readOnly: true
)
}

error in user()

Thanks,

kiran