Trying to create a record in custommodule1 - error 4401

Trying to create a record in custommodule1 - error 4401

Here's my code:

RSmap=map();
RSmap.put("referral source name","FN");
response = zoho.crm.create("custommodule1",RSmap,1);
info response;

Here is response:

Executed Successfully
Log messages:
{"message":"Unable to populate data, please check if mandatory value is entered correctly.","code":"4401"}
This custom module has only one mandatory field - Referral Source Name. I've tried all Caps, underscores, I've pored over the generous documentation twice and that took a minute. So what's wrong with this code?