serverless-plugin-warmup: Cannot set property 'warmUp' of undefined

I’m getting the following error:

{ "errorType": "TypeError", "errorMessage": "Cannot set property 'warmUp' of undefined", "trace": [ "TypeError: Cannot set property 'warmUp' of undefined", " at Module.<anonymous> (/var/task/_warmup/webpack:/_warmup/index.js:9:1)", " at Module.module.exports (/var/task/_warmup/index.js:146:30)", " at __webpack_require__ (/var/task/_warmup/webpack:/webpack/bootstrap:19:1)", " at /var/task/_warmup/webpack:/webpack/bootstrap:83:1", " at Object.<anonymous> (/var/task/_warmup/index.js:87:10)", " at Module._compile (internal/modules/cjs/loader.js:776:30)", " at Object.Module._extensions..js (internal/modules/cjs/loader.js:787:10)", " at Module.load (internal/modules/cjs/loader.js:653:32)", " at tryModuleLoad (internal/modules/cjs/loader.js:593:12)", " at Function.Module._load (internal/modules/cjs/loader.js:585:3)" ] }

Any ideas? It’s proving challenging to debug.