question: importing routes from kernel filtered by function with prefix match

[Kai]

Dear experienced bird experts! :)

While trying to setup my first more complex bird config I cannot determine the error I made that impede my config from working.

Excerpt from the config:
  table kern_wimz;
  function is_wimz_self_nets() {
      return (net ~ [10.56.0.0/16+,
                     10.37.0.0/16+]);
  }
  # import only self from kernel, export anything but wi&mz - remark: exporting done by other kernel protocoll instances
  protocol kernel kern_import {
      scan time 30;
      import where is_wimz_self_nets();
      export none;
      table kern_wimz;
      kernel table 254;   # main
  };

I also added a - seemingly neccessary:
  table bla;
  protocol device {
      scan time 30;
      table bla;
  };

(Complete config: https://github.com/freifunk-mwu/technik-meta/wiki/birdconfig )

In the main kernel routing table (#254) one can find - among others - the two prefixes 10.37.0.0/18 and 10.56.0.0/18 . I expected these two to be imported into kern_wimz. From there they are going to be transported to other tables by the pipe protocol.

But kern_wimz remains empty. :(

I consulted the documentation multiple times already, but I cannot pinpoint the mistake I made. Could anyone please provide me with a helpful hint?

Best regards, Kai