Languages such as Czech exhibit negative concord} a requirement for any neg-word (e.g. 'nobody') in a clause to be accompanied by sentential negation. Theoretical treatments see it as either a case of syntactic agreement or subsume it under NPI licensing. Interestingly, in the processing of both agreement and NPIs, the inclusion of a distractor element can induce comprehenders to accept otherwise ungrammatical sentences. We tested whether such grammaticality illusions also arise with negative concord. We ran a speeded acceptability judgement experiment and an untimed one with native Czech speakers to see whether the inclusion of an irrelevant negated verb in a relative clause caused them to accept an ungrammatical sentences an unlicensed neg-word. This tendency was clearly seen in the speeded experiment, but not when participants had enough time for their answers. Our results thus show that there is indeed an effect—the negative concord illusion.