Problem with type mismatch

Hello. I'm trying to grab a field value in a CRM record by first looking up the record by name. It seems I must then lookup by record ID so I can have access to custom fields. Here is my short sequence that is getting an error:

productRecord = zoho.crm.searchRecords("Products","(Product Name|=|Magnetic Attraction)");

productId = productRecord.get("Product_ID");

productDetails = zoho.crm.getRecordById("Products",productId);

audioUrl=productDetails.get("Audio");

The second line has produced the following error message:

Mismatch data type for function get at argument index 1 expected data type BIGINT found dataType STRING

I'm not familiar with the structures involved but my understanding is that the "searchRecords" function only returns some core fields. I think I need to extract the record ID from that and then use that number "Product_ID" to retrieve the full record, after which I can access the custom field "Audio".

Please let me know where I have gone astray....

Thanks,

Doug