COLLECT-UML is an intelligent tutoring system that teaches Object-Oriented design using Unified Modelling Language (UML). UML is one of the most popular techniques used in the design and development of Object-Oriented systems nowadays. The Constraint-Based Modelling (CBM) has been used successfully in several systems and they have proved to be extremely effective in evaluations performed in real classrooms. In this paper, we present our experiences in implementing another constraint-based tutor, in the area of Object-Oriented design. We present the system's architecture and functionality and describe the results of a preliminary study with postgraduate students who interacted with the system as part of a think-aloud study. Participants felt that using the system helped them improve their UML knowledge. A full evaluation study is planned for May 2005, which aims to evaluate the interface and the effect of using the system on students' learning.