Mocap Data invalid

Hi all ! I’m new to Px4, for a school project, I need to fly a drone indoor using a mocap Optitrack system. At the moment, I have an error which tells me that the Mocap data is invalid.

INFO [local_position_estimator] [lpe] mocap data invalid. eph: 0.977565 epv: nan

Do you know where that might come from ?
I’m using a pixhawk 4 mini, Ros Melodic and QAV250 drone