Conversational Agents (CAs) have become a new paradigm for human-computer interaction. Despite the potential benefits, there are ethical challenges to the widespread use of these agents that may inhibit their use for individual and social goals. However, besides a multitude of behavioral and design-oriented studies on CAs, a distinct ethical perspective falls rather short in the current literature. In this paper, we present the first steps of our design science research project on principles for a value-sensitive design of CAs. Based on theoretical insights from 87 papers and eleven user interviews, we propose preliminary requirements and design principles for a value-sensitive design of CAs. Moreover, we evaluate the preliminary principles with an expert-based evaluation. The evaluation confirms that an ethical approach for design CAs might be promising for certain scenarios.