Its likely that you're missing some other errors that happened earlier.
Inventory Step is the last step in the
integrity check stage and the migration tool runs all steps in the integrity check stage, even if one of them throws an exception. Problems occurring anywhere during the
integrity check stage get reported at the end of it, not the step at which they occur.
If you look at at \Migration\Mode\Data::runIntegrity, you can see it has a foreach loop, which runs all steps in the integrity check stage, but it only checks if these steps completed successfully after the loop and throws the
Integrity Check Failed exception if any one of the steps failed.
Chances are that you'll have red errors further up the output. As suggested in the comments above you need to carefully review your
/var/migration.log file to find out what step in the integrity check is actually failing and why.