An engineer is building a text generation system. At a specific step, the system has calculated the following probabilities for the top 3 most likely next words: 'the' (0.5), 'a' (0.2), and 'one' (0.1). The engineer's code directly uses these probabilities to sample the next word from this restricted set. However, they notice their program sometimes fails to select any word at all for this step. Based on the principles of probability distributions for sampling, identify the fundamental error in the engineer's approach and explain why it leads to the observed failure.