Cannot read property 'elevation' of null

[cerealexx]

The documentation says you can pass null or an empty string to the shadow directive to remove the shadow. I'm dynamically passing a number value or null depending on the platform with this method:

public iosShadow(level: string): number {
    if (this.platform === 'ios') {
        return Number(level);
     }
    return null;
}

And view:

<StackLayout [shadow]="iosShadow(2)">
    ...
</StackLayout>

Though I always get this error when passing null: Cannot read property 'elevation' of null