Error in Escalation Timer (ERROR:EVAL:@reason=index)

I'm receiving the following error when configuring an escalation timer: "ERROR:EVAL:@reason=index". I saw a corresponding knowledge base article indicating that I should not have a load() in a rule within the escalation timer, but even after removing that load(), and even directly inputting my expression into the expression editor, I'm still receiving this error. Has anyone experienced this issue before or have any insight? Code is below:

caladdhours(
rule!XXXX_getXXXX(
caseId: ri!case.id,
statusId: ri!case.statusTypeId,
pagingInfo: a!pagingInfo(
startIndex: 1,
batchSize: 1,
sort: a!sortInfo(field: "createdOn", ascending: true)
)
).data.createdOn,
rule!XXXX_getREFXXXX(
id: ri!case.xxxxTypeId
).data.slaBusHrs,
"XXXX"
)