SSS_INVALID_TYPE_ARG when calling http.createSecureString

I am trying to access an API Secret from a SuiteScript. Everything I've read is that in its simplest form, the call is something like:

let secureString = https.createSecureString({input: "{custsecret_id}",});

However, when I try that, I get an SSS_INVALID_TYPE_ARG error. Here's a screenshot with my API Secret key. It is allowed for all scripts and all domains.

Boiling everything down to the most simple snippet still fails.

Code (run in debugger):

require(["N/https"], function (https) {
let secureString = https.createSecureString({input: "{custsecret_i1_cw_api_test}"});
let anotherString = "test";
// code using secureString to follow
});

Error: