What does this mean degenerates mass distribution

What this error means, im trying to model linear actuator but it always gives me this error has a degenerate mass distribution on its follower side.

Thanks

[Walter Roberson]

https://www.mathworks.com/matlabcentral/answers/690940-degenerate-mass-distribution-on-follower-side-of-a-joint

https://www.mathworks.com/matlabcentral/answers/244110-model-walking-body-in-simulink#answer_193147