Null guard not working for c# 8 Nullable #72

I'm testing library for c# 8 support but it looks like it is not working

public static string Test(string? text) { text.MustNotBeNull(); return text.Substring(0, 1); }

With proper implementation there should be no warrning for text but currently I have:

"Warning CS8602 Dereference of a possibly null reference"

Check is only woring when return value is used

public static string Test(string? text) { var x = text.MustNotBeNull(); return x.Substring(0, 1); }

but I would not like to create separate variable only for this