// A *self-contained* demonstration of the problem follows... // Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc. type EventType = 'click' | 'dblclick' const handlerMap: { [P in EventType]?: any[] } = {} function addHandler<P extends EventType>(evType: P) { const handlerList = handlerMap[evType] || [] handlerList.push({}) // Error here: Object is possibly 'undefined' handlerMap[evType] = handlerList }