The emergence of artificial intelligence (AI) has heightened interest in personalizing computer assisted learning (CAL) programs to tailor their instruction to individual students. Despite the proliferation of AI-driven CAL programs, evidence for their effectiveness remains limited. We present findings from a field experiment in rural China examining whether an AI-driven personalized CAL program improves student achievement. We randomly assign 8647 students from 315 primary school classes to one of three treatment arms: (i) AI-CAL, (ii) non-personalized CAL (active control), and (iii) non-CAL educational activities (pure control). Results indicate that AI-CAL does not significantly improve achievement, with estimates precise enough to rule out non-trivial positive effects. This finding holds across the difficulty of assessment items and across the baseline achievement of students. If anything, AI-CAL might be less effective than non-personalized CAL for students in the middle tercile of baseline achievement. These findings call for caution when scaling AI-driven educational technologies.