Robotic Process Automation (RPA) aims to automate rule-based business process tasks by software robots (bots) mimicking human interactions. Despite the partial automation achieved with RPA, humans still need to interact with the bots, which requires appropriate user interfaces. However, existing RPA research has not evaluated RPA from a software-ergonomic perspective so far and no corresponding user interface design guidelines exist. The objective of this paper is to evaluate the usability of RPA bots in industry and to provide user interface design guidelines to bot developers. The results we obtained from 50 questionnaires filled by RPA users indicate that both the input/output and the dialogue interfaces of RPA need to be improved, especially regarding error tolerance, perceptibility, directability of user's attention, suitability for the task, and availability. Finally, we derive seven guidelines for designing the user interface of RPA bots. Potential improvements include, among others, the quality of error messages, the efforts for error handling, and the monitoring of the current status of the tasks assigned to the bot.