ValidationException: ExpressionAttributeValues contains invalid key

Question

I'm trying to filter the results of a scan of a DynamoDB table using a FilterExpression in my query. I'm using a Lambda function deployed on API Gateway on AWS.

Function Querying the Endpoint

// Scan the table for entries and use data to populate HTML table.
async function scanTable(){
  
    var query = {
        TableName: "bookings",
        ProjectionExpression: "id, username, start_booking, end_booking",
        FilterExpression: "username = :u",
        ExpressionAttributeValues: {":u":"human@gmail.com"}
    };

    try{      
        const response = await axios({ method: 'get', url: `${url}/bookings`, params: query });    
    }catch(err){
        console.log(err);
    }
}

Lambda Function

'use strict'
const AWS = require('aws-sdk');


exports.handler = async (event, context) => {
    const documentClient = new AWS.DynamoDB.DocumentClient({convertEmptyValues: true});

    let responseBody = "";
    let statusCode = 0;

    const params = (event.queryStringParameters);
    // console.log(params);
    
    try{
        const data = await documentClient.scan(params).promise();
        responseBody = JSON.stringify(data.Items);
        statusCode = 200;
    }catch(err){
        responseBody = `Unable to get the bookings: ${err}`;
        statusCode = 403;
    }
    
    console.log("Response Body: " + responseBody);

    const response = {
        statusCode: statusCode,
        headers: {
            "Content-Type": "application/json",
            "Access-Control-Allow-Origin": "*"
        },
        body: responseBody
    };
    
    

    return response;
}

The complete error I receive is

ValidationException: ExpressionAttributeValues contains invalid key: Syntax error; key: "11"

where the key value at the end of the error (key:"11") changes somewhat dependent on the value assigned in ExpressionAttributeValues. I've tried to follow the FilterExpression example in the DynamoDB Docs for the scan function to no avail. Here is another example.

Here is a similar question from the AWS forums.