There is a growing belief that scalable and low-cost AI assistance can improve firm decision-making and economic performance. However, running a business involves a myriad of open-ended problems, making it hard to generalize from recent studies showing that generative AI improves performance on well-definedwriting tasks. In our field experiment with 640 Kenyan entrepreneurs, we assessed the impact of AI-generated advice on small business revenues and profits. Participants were randomly assigned to a control group that received a standard business guide or to a treatment group that received a GPT-4-powered AI business mentor via WhatsApp. We are unable to reject the null hypothesis that generative AI access has no impact, but are able to rule out the large effect sizes reported by other studies of generative AI’s economic impact. Our overall null result masks treatment effect heterogeneity with respect to the baseline business performance of the entrepreneur: our point estimates suggest that high performers benefited by just over 15% from AI advice, whereas low performers did about 8% worse with AI assistance. Exploratory analysis of WhatsApp interaction logs shows that both groups sought the AI mentor’s advice, but that low performers did worse because they sought help on more challenging business tasks. Our findings highlight the potential and limitations of generative AI to enable entrepreneurs across the globe.