Flux left join on empty table - influxdb

Question

I'm looking to join two data streams together but receive the following error from Influx:
error preparing right side of join: cannot join on an empty table
I'm trying to build a query which compares the total sales by a store this month compared to last month. If the store has no sales this month then I don't want it to show. Below is a basic example of my current query.
import "join"
lastMonth = from(bucket: "my-bucket")
|> range(start: 2022-10-01, stop: 2022-11-01)
|> filter(fn: (r) => r._measurement == "transaction")
|> pivot(rowKey: ["_time"], columnKey: ["_field"], valueColumn: "_value")
|> group(columns: ["storeId"], mode: "by")
|> reduce(
fn: (r, accumulator) => ({
storeId: r.storeId,
amount: accumulator.amount + (r.totalAmount - r.refundAmount)
}),
identity: {
storeId: "",
amount: 0.0
}
)
from(bucket: "my-bucket")
|> range(start: 2022-11-01, stop: 2022-12-01)
|> filter(fn: (r) => r._measurement == "transaction")
|> pivot(rowKey: ["_time"], columnKey: ["_field"], valueColumn: "_value")
|> group(columns: ["storeId"], mode: "by")
|> reduce(
fn: (r, accumulator) => ({
storeId: r.storeId,
amount: accumulator.amount + (r.totalAmount - r.refundAmount)
}),
identity: {
storeId: "",
amount: 0.0
}
)
|> join.left(
right: lastMonth,
on: (l, r) => l.storeId == r.storeId,
as: (l, r) => ({
storeId: l.storeId,
thisMonthAmount: l.amount,
lastMonthAmount: r.amount
})
)
How can I achieve this in Flux without encountering this issue?