Error: Indeterminant linear system

[Flacedoo]

Hello,
I am trying to use the aprilslam library to localize my camera inside a ring of apriltags. Unfortunatly, I get from time to time the following error, and aprilslam crashes and starts from new:

[ERROR] [1500970493.767645733]: //duo3d/left: 
Indeterminant linear system detected while working near variable
7782220156096217104.

Thrown when a linear system is ill-posed.  The most common cause for this
error is having underconstrained variables.  Mathematically, the system is
underdetermined.  See the GTSAM Doxygen documentation at
http://borg.cc.gatech.edu/ on gtsam::IndeterminantLinearSystemException for
more information.

I wasn't able to tell when exactly this error occured. It even occured when I moved the camera slowly from one tag to another without losing sight of one of them..

Thanks for your help!