As ROL said, it is difficult to know what the instructor meant by "safe" without seeing your work. If someone told me that, I would assume that they were encouraging me to stretch my boundaries and photograph things that were less comfortable. For example, I am quite comfortable photographing things like derelict buildings and vehicles. Adding a live model to those scenes would make them less "safe" to me; they would cause me to stretch my boundaries. But you really should ask your instructor what he/she specifically meant.