On the CLI this works:

aws dynamodb query 
    --table-name BatchTable 
    --index-name completed_index 
    --key-condition-expression "completed = :completed" 
    --expression-attribute-values  '{":completed":{"N":"0"}}'

Result:

{
    "Items": [
        {
            "completed": {
                "N": "0"
            },
[snip]

But in Lambda this doesn’t:

import { DynamoDBClient } from '@aws-sdk/client-dynamodb';
import { DynamoDBDocumentClient, GetCommand } from '@aws-sdk/lib-dynamodb';
const ddbClient = new DynamoDBClient({});
const ddbDocClient = DynamoDBDocumentClient.from(ddbClient);

const ddbCommand = new GetCommand({
    TableName: 'BatchTable',
    IndexName: 'completed_index',
    KeyConditionExpression: 'completed = :completed',
    ExpressionAttributeValues: {
        ':completed': '0',
    },
});

const ddbResponse = await ddbDocClient.send(ddbCommand);

Result:

1 validation error detected: Value null at 'key' failed to satisfy constraint: Member must not be null

I can’t for the life of me figure out why.