Apache Nifi: Cannot convert value <field> of type class org.apache.nifi.serialization.record.MapRecord to Long for <field>

[#1]

Hello everyone! I'm new to Apache Nifi and I'm trying to get some data from a database (db1), perform a lookup using another another database (db2) and then loading this new data into a redshift. The problem is that my LookUp Process fails.

The incoming data from the db1 looks like the following:

{
transacition_id: 123
customer_id: 321
amount: U$10.0
} 

On the db2 I have this:

{
pk_customer: 001
src_sys_key: 321
customer_name: "example"
}

What I would like to do is get the pk_customer from the db2 by matching customer_id with src_sys_key.

The current Nifi flow:

LookupRecord settings:

And finally the LookupService (Database Record Lookup):

Getting the following error:

 

Can anyone help me, please?

Thank you in advance!