Web.config httpCompression section is ignored

Question

I am trying to add a mimeType in web.config -> httpCompression -> dynamicTypes. What i found was that it had no effect and it seems that the section is being ignored. For example: Adding this element gives no error

<dynamicTypes>
    <addfail mimeType="someType" invalidAttribute="true" />
</dynamicTypes>

Or this

<dynamicTypes>
    qwerty
</dynamicTypes>

But when i change this for example:

<system.web>
    <customErrorsssssss mode="Off" />
</system.web>

I get an Internal server error.

What am i missing?