covariates: Fix bug in covariate_gatherer
covariates: Fix bug in covariate_gatherer

Check the correct key when determining if the deletion key is invalid.

NVlabs/firepony,NVlabs/firepony,broadinstitute/firepony,broadinstitute/firepony,NVlabs/firepony,broadinstitute/firepony,NVlabs/firepony,broadinstitute/firepony