Gamerule minecartMaxSpeed can't be set through functions

Trying to set minecartMaxSpeed through a function such as

gamerule minecartMaxSpeed 1000

fails, even when the experimental minecart improvements feature flag is enabled.

I tried a workaround using a macro, in case the experimental pack is activated after the function is loaded, but this didn't work.

What I expected to happen was...: it to be set like any other gamerule would be
What actually happened was...: the function failed to load
Steps to Reproduce:

1. Create a world with the attached datapack
2. Try running the either of these commands (they are equivalent) 
   /function rennen:set_macro_post {key_1:"minecartMaxSpeed"}
   /function rennen:set_macro_pre
3. View to log file to see why rennen:set_direct, which just does: 

   gamerule minecartMaxSpeed 1000 

   was unable to load

Also attached is a related log file