Sort not working for null values from an endpoint

We have a grid, the values come from an endpoint.

Some of those values are "null". The sort method is not working properly. Looks like the table is not able to handle null values.

I've tried to add valuePrepareFuntion like this:
`valuePrepareFunction: (value) => { return value == null ? "" : value }`

but it was not replacing the value, it was keeping null value instead of "" (empty String)

This is what I'm using:

angular/cli: 1.4.0
node: 6.11.1
os: darwin x64
angular/animations: 4.3.6
angular/cdk: 2.0.0-beta.10
angular/common: 4.3.5
angular/compiler: 4.3.5
angular/core: 4.3.5
angular/forms: 4.3.5
angular/http: 4.3.5
angular/material: 2.0.0-beta.10
angular/platform-browser: 4.3.5
angular/platform-browser-dynamic: 4.3.5
angular/router: 4.3.5
angular/cli: 1.4.0
angular/compiler-cli: 4.3.5
angular/language-service: 4.3.5
typescript: 2.3.4