Why can I search in the Lead column, but not on the Lead_ID column of CustomModule1?

Why can I search in the Lead column, but not on the Lead_ID column of CustomModule1?

I've created a CustomModule1 with a Lookup field for Leads.
Successfully, I've added a record.

This is the request URL:
  1. https://crm.zoho.com/crm/private/xml/CustomModule1/searchRecords?authtoken=WORKING_KEY&scope=crmapi&criteria=(Lead_ID:1820378000000097468)

When I use the API method searchRecords, I get an error:
4832: API call cannot be completed as the Criteria parameter contains empty field name.
For testing purposes, I changed the Lead_ID to 'fewgd'. It returns the same error.

When I change the criteria to another field name (also related to the same lookup field): lead, it returns the record(s) requested.
  1. https://crm.zoho.com/crm/private/xml/CustomModule1/searchRecords?authtoken=WORKING_KEY&scope=crmapi&criteria=(Lead:Carsten%20de%20la%20Fonteijne)
The attachment Zoho.PNG shows a part of the returning XML.

My question is, why is this not possible?
Or am I just doing something wrong?

This is also true for Account and Account_ID