Error parsing XML: unbound prefix in custom view

I tried customizing a view from a library I'm using for my android app. The default xml code for this looks like this:

<it.gmariotti.cardslib.library.view.CardView
            android:id="@+id/carddemo"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="12dp"/>

This works without any problems. But when I add a new line in the element, to use my custom layout, I get an error in the opening line, that you can see in the title of this question: This is the line I was adding in the end:

        card:card_layout_resourceID="@layout/custom_layout" />

This is also exactly the code that was used as an example in the docs of the library.