AWS PUT request met with "Provided key element does not match schema." - node.js

Question

(Edited to incorporate comments)
So I apologize in advance for the long question. I don't know how else to ask it.
I'm trying to finish up a full-stack web app using React, Node, and DynamoDB. POST and GET requests are working fine, but I'm stuck on PUT. My mock PUT request works fine, but once I try it from the front end in React, I get the error mentioned in the title. I'll show the back end code first, then the mock update, and then the front end.
import handler from "./libs/handler-lib";
import dynamoDb from "./libs/dynamodb-lib";
export const main = handler(async (event, context) => {
const data = JSON.parse(event.body);
const params = {
TableName: process.env.tableName,
Key: {
userId: event.requestContext.identity.cognitoIdentityId,
activityId: event.pathParameters.activityId
},
UpdateExpression: "SET title = :title, activityType = :activityType, activityRoutine = :activityRoutine, activityComment = :activityComment",
ExpressionAttributeValues: {
":title": data.title || null,
":activityType": data.activityType || null,
// ":activityRoutine": data.activityRoutine == '' ? "None" : data.activityRoutine,
// ":activityComment": data.activityComment == '' ? "None" : data.activityComment
":activityRoutine": data.activityRoutine || null,
":activityComment": data.activityComment || null
},
ReturnValues: "ALL_NEW"
};
await dynamoDb.update(params);
return { status: true };
This mock update event works without issue:
{
"body": "{\"title\":\"test\",\"activityType\":\"testing\",\"activityRoutine\":\"\",\"activityComment\":\"\"}",
"pathParameters": {
"activityId": "long-alphanumeric-id"
},
"requestContext": {
"identity": {
"cognitoIdentityId": "us-east-and-so-on"
}
}
}
But this code, which produces the exact same Javascript object as the mock, is not okay with AWS:
function saveActivity(activity) {
try {
return API.put("activities", `/activities/${id}`, {
body: activity
});
} catch(e) {
console.log("saveActivity error:", e);
}
}
async function handleSubmit(event) {
event.preventDefault();
setIsLoading(true)
try {
await saveActivity({
title: title, activityType: activityType, activityRoutine: activityRoutine, activityComment: activityComment
// "key": {userId: userId, activityId: activityId}
// "pathParameters": {"id": activityId},
// "requestContext": {"identity": {"cognitoIdentityId": userId}}
});
} catch(e) {
console.log(e)
setIsLoading(false)
}
}
If anyone needs to see more of the code, I'm happy to share, but I figured this question is already getting very long. Any code you see commented out has been tried before without success.
I'd also be happy if someone could point me in the right direction as far as the AWS documentation is concerned. I've been going off of a tutorial and modifying it where need be.
Any help is appreciated!