How to use Google Chart when pivot table is in Flat mode?

Hi, I am able to show a pie chart with classic type of pivot table, but when I set the type to flat by default the chart was unable to show.
Here is the error

core.js:15724 ERROR TypeError: Cannot read property 'members' of undefined
    at A4n.composeData (webdatarocks.js:6)
    at A4n.getData (webdatarocks.js:6)
    at R1n.getData (webdatarocks.js:6)
    at I5v.getData (webdatarocks.js:6)
    at Object.PivotApi.W1C.getData (webdatarocks.js:6)
    at Object.WebDataRocksGooglecharts.getData (webdatarocks.googlecharts.js:31)
    at ByProductComponent.push../src/app/pages/product-report/by-product/by-product.component.ts.ByProductComponent.createGoogleChart (by-product.component.ts:111)
    at ByProductComponent.push../src/app/pages/product-report/by-product/by-product.component.ts.ByProductComponent.onReportComplete (by-product.component.ts:99)
    at SafeSubscriber._next (outlet.service.ts:387)
    at SafeSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.SafeSubscriber.__tryOrUnsub (Subscriber.js:196)

Thank you.