Hi Jelle,
I have done a little more work on this today and found, as usual, the issue was with my implementation 
The dynamic value I was retrieving from my API call “favicon.data[0].$value” which was provided by the Wappler data binding selection was not actually giving me a value, it should have been “favicon.data[0]”
It now works and my day of trying to find an error has taught me loads about extensions 
Thanks, Paul.