enumeration error

Hi folks, 

Following is the code that gives me an error when a text or null value is entered. 

Error: 

Interface Definition: Expression evaluation error: An error occurred while executing a save: Expression evaluation error at function 'enumerate' [line 357]: A null parameter has been passed as parameter 1.

Code:

a!integerField(
label: "Number of tests",
labelPosition: "ADJACENT",
requiredMessage: "Number of tests cannot be left blank.",
required: true(),
value: ri!cdt_PS_processDetails.nEstimatedTests,
saveInto: {
ri!cdt_PS_processDetails.nEstimatedTests,
a!save(
ri!cdt_PR_trackingDetails,

a!forEach(
items: enumerate(
ri!cdt_PS_processDetails.nEstimatedTests
),
expression: 'type!{urn:com:appian:types:xyz}AP_TRACKING_DETAILS'(
nMasterTicketId: ri!cdt_TicketDetails.MASTER_TICKET_ID
)
)
)
},
refreshAfter: "UNFOCUS",
validations: {}
)

Is there a way to resolve this error?

TIA :)