What's the difference between a is string and a !is string in c# [duplicate]

I need to understand the meaning of !is in c#. I understand the is keyword. I expected that a !is b would be the same as !(a is b). However, after testing, I found out that this is not the case. I just need to know what it means and when it may be useful.

I have checked the documentation for is.

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/is

From the navigation, it appears the only type testing keyword is is. There's no obvious reference to !is. I can find none in the same page as is, yet the compiler doesn't fail when I use !is. (Is this a bug?).

previousStageResult.Data is defined as object.

// this
var previousResultIsInvalid = previousStageResult.Data !is Dictionary<string, int>;

// returns the same thing as
var previousResultIsInvalid = previousStageResult.Data is Dictionary<string, int>;

// What I expect:
var previousResultIsInvalid = previousStageResult.Data !is Dictionary<string, int>;

// should be the same as
var previousResultIsInvalid = !(previousStageResult.Data is Dictionary<string, int>);

// if it is even valid at all

Thank you.