Expression error: Null Arguments are Invalid) (Data Inputs)

Hi All,

I am getting expression error when trying to pass the below rule in the input a user input task. I am passing the correct PV value in every step of my flow. Please suggest if there is anything wrong in this rule. Thanks in advance. 

Expression error: An error occurred while evaluating expression: =rule!REGAL_cdt_getDocumentsFromFolder(pv!caseDocFolder) (Expression evaluation error in rule 'REGAL_cdt_getdocumentsfromfolder' at function 'getdocumentinternalidsfromfolder' [line 2]: Null Arguments are Invalid) (Data Inputs)

REGAL_cdt_getdocumentsfromfolder Rule Code:

load(
local!documentsFromFolder: getdocumentinternalidsfromfolder(
ri!folder,
false()
),
a!forEach(
items: local!documentsFromFolder,
expression: cast(
'type!{urn:com:appian:types}REGAL_documents'(),
{
documentId: tointeger(
fv!item
),
documentName: document(
tointeger(
fv!item
),
"name"
),
createdBy: document(
fv!item,
"lastUserToModify"
),
createDateTime: document(
fv!item,
"dateCreated"
)
}
)
)
)