changed_materials error on adjoint run

[poutine-dejeuner]

This happense during the adjoint run of a call to an mpa.OptimizationProblem

Starting adjoint run...                                                              
MPB solved for frequency_1(-0,645833,0,0) = 0,28956 after 13 iters                   
MPB solved for frequency_1(-1,58873,0,0) = 0,62126 after 9 iters                     
MPB solved for frequency_1(-1,65888,0,0) = 0,6455 after 7 iters                      
MPB solved for frequency_1(-1,65887,0,0) = 0,645497 after 2 iters                    
python: step_db.cpp:40: void meep::fields::step_db(meep::field_type): Assertion `changed_materials' failed.                                                               
fish: Job 1, 'python Tutorial_3_lsf_settings.…' terminated by signal SIGABRT (Abort)