It seems like that the original _sbti_load()_ function from the library just returns NULL instead of actual pointer. You may can check it using the _repr_ proc: echo data.repr
As far as I can see, the _sbti_load()_ function receives a file name so may be the issue is in the test suite, in the test file names declarations. You can also investigate the problem by inspecting produced C files in the _nimcache_ directory.