Walker was tested at the final day of the Pan American Games in Lima. Benzoylecgonine is a metabolite, or breakdown product, of cocaine and is on the Prohibited Substance List. The CAS decided that both Walker's performance and that of the Canadian team should be dropped.

Exactly what the penalty for Walker will be has yet to be determined. The FEI will decide on the sanction. The rider has been suspended since November 8, 2019.