Lie detectors don't work
Lie detectors are not accurate. They are little better than a coin toss.
Research indicates that an innocent black candidate is more likely to induce a false positive polygraph reading than an innocent white candidate.
In a test examining more than 1,100 subjects’ polygraph results, only 23.5% of innocent black candidates were correctly identified as being non-deceptive. By contrast, 36.9% of innocent white participants were correctly identified. In the Modified General Question Test (a variant question structure), only 14.6% of black participants were correctly identified as giving non-deceptive answers, compared to 33.3% of whites. In polygraph tests administered to FBI applicants between October 2008 and June 2010, Asians, Hispanics, Native Americans and Pacific Islanders were also overrepresented in the failed applicants dismissed for failing the test.  Polygraph detectors not only introduce new avenues for racial bias in the justice system, more dangerously, the shroud it in the veil of science and authenticity. They take human prejudices, wrap them up in science and present them as an “objective technological finding”. 
The test itself is not racially biased. It is the vessel through which examiners can project their beliefs. Therefore, it is no more vulnerable to racial biased than a jury of the defendant’s peers might be. The existing judicial system is racially biased; however, this is because racism is still in existence in wider society. No legal tool will be protected from individual prejudice, be it trial by jury, lie detectors, expert testimony, or non-jury trials overseen by a judge, as long as the margin for human error exists. Given a polygraph test’s proven ability to determine lie from truth at a better rate than behavioral analysis and guesswork, it’s evidence should be admitted in court under the understanding that any method to determine guilt is vulnerable to individual prejudice, but until we can remove the margin for human error, we cannot escape this.
Enter the framing of the argument here ...
[P1] Black candidates and defendants of other minority races achieve false-positive readings at a higher rate than white candidates. [P2] If lie detector tests were admissible in court, this would make the legal system increasingly biased against minority races. [P3] Therefore we cannot allow lie detector evidence to be admissible in court.
[Rejecting P3] No system for determining guilt from innocent is protected from human error. While human error exists, there will be room for an individual's racial prejudices to influence the outcome. Lie detectors are the best we have for now. Therefore, they should be admissible in court.
Enter more information about the argument here ...