Why TypesScript let use key which is does not specified?

Here I have this line:

const _quantity = startPaymentFrontend?.products[productSlug]?.invoiceData?.[key2]

This here

startPaymentFrontend?.products[productSlug]

is a StartPaymentProduct

export interface StartPaymentProduct {    pricingOptionId?: string    passTypeId?: string    quantity: number    eventTimes?: { [eventTimeId: string]: StartPaymentProductEventTime }    invoiceItems?: { [key: string]: number }    teamSlug: string}

Even developer environment recognize it.

which does not have invoiceData only invoiceItems.

Why running yarn build does not raise error on it?

This here is the tsconfig.json

{"compilerOptions": {"target": "es5","lib": ["dom", "dom.iterable", "esnext"],"allowJs": true,"skipLibCheck": true,"strict": true,"forceConsistentCasingInFileNames": true,"noEmit": false,"esModuleInterop": true,"module": "esnext","moduleResolution": "node","resolveJsonModule": true,"isolatedModules": true,"jsx": "preserve","sourceMap": true,"importHelpers": true,"outDir": "./built"    },"include": ["app/**/*", "index.d.ts", "index2.d.ts"],"exclude": ["node_modules"]}

How can I force to raise error in those cases?!